12.2A: Genes as the Unit of Heredity - Biology

12.2A:  Genes as the Unit of Heredity - Biology

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Genes exist in pairs within an organism, with one of each pair inherited from each parent.

Learning Objectives

  • Describe the structure of a gene and how offspring inherit genes from each parent

Key Points

  • A gene is a stretch of DNA that helps to control the development and function of all organs and working systems in the body.
  • Genes are passed from parent to offspring; the combination of these genes affects all aspects of the human body, from eye and hair color to how well the liver can process toxins.
  • A human will inherit 23 chromosomes from its mother and 23 from its father; together, these form 23 pairs of chromosomes that direct the inherited characteristics of the individual.
  • If the two copies of a gene inherited from each parent are the same, that individual is said to be homozygous for the gene; if the two copies inherited from each parent are different, that individual is said to be heterozygous for the gene.

Key Terms

  • gene: a unit of heredity; the functional units of chromosomes that determine specific characteristics by coding for specific proteins
  • chromosome: a structure in the cell nucleus that contains DNA, histone protein, and other structural proteins
  • genetics: the branch of biology that deals with the transmission and variation of inherited characteristics, in particular chromosomes and DNA

Pairs of Unit Factors, or Genes

Mendel proposed that paired unit factors of heredity were transmitted faithfully from generation to generation by the dissociation and reassociation of paired factors during gametogenesis and fertilization, respectively. After he crossed peas with contrasting traits and found that the recessive trait resurfaced in the F2 generation, Mendel deduced that hereditary factors must be inherited as discrete units. This finding contradicted the belief at that time that parental traits were blended in the offspring.

A gene is made up of short sections of DNA that are contained on a chromosome within the nucleus of a cell. Genes control the development and function of all organs and all working systems in the body. A gene has a certain influence on how the cell works; the same gene in many different cells determines a certain physical or biochemical feature of the whole body (e.g., eye color or reproductive functions). All human cells hold approximately 21,000 different genes.

Genetics is the science of the way traits are passed from parent to offspring. For all forms of life, continuity of the species depends upon the genetic code being passed from parent to offspring. Evolution by natural selection is dependent on traits being heritable. Genetics is very important in human physiology because all attributes of the human body are affected by a person’s genetic code. It can be as simple as eye color, height, or hair color. Or it can be as complex as how well your liver processes toxins, whether you will be prone to heart disease or breast cancer, and whether you will be color blind.

Genetic inheritance begins at the time of conception. You inherited 23 chromosomes from your mother and 23 from your father. Together they form 22 pairs of autosomal chromosomes and a pair of sex chromosomes (either XX if you are female, or XY if you are male). Homologous chromosomes have the same genes in the same positions, but may have different alleles (varieties) of those genes. There can be many alleles of a gene within a population, but an individual within that population only has two copies and can be homozygous (both copies the same) or heterozygous (the two copies are different) for any given gene.

Nohrina Unit 7 Cell division and Heredity

Meiosis Meiosis is a process for producing sex cells. These cells can create a distinctive genotype through the contribution of two parents’ DNA. Meiosis create Gametes.[CITATION Epearl l 2057 ].

Meiosis involves two divisions and starts in the same way as mitosis.

Meiosis I consists of Prophase I, Metaphase I, Anaphase I and Telophase I.

DNA replicates and curls up to for double armed chromosomes. After replication, the 46 chromosomes arrange into 23 pairs. Chromosomes in a pair contain the same features.

In the first division, the pair split up and the chromosomes move to opposite poles of the cell. New cell consists of only 1 pair of 23 different types, this means it becomes a mixture of parental chromosomes, but only half number of chromosomes[CITATION Epearl l 2057 ].

Meiosis II consists of Prophase II, Metaphase II, Anaphase II and Telophase II.

In second division each homologous chromosome splits into half and one arm ends up in each new cell where it ends up having 4 daughter cells. These cells are genetically different from each other because the chromosomes all get mixed up during the meiosis and each gamete only gets half of them randomly. Daughter cells are known as diploid cells which then divide again to become haploid cells. This indicated that they have half number of chromosomes.[ CITATION Par11 l 2057 ].

Just as in mitosis, before

The daughter cells then go

And meiosis is that meiosis

Creates new

Cells with a wide variety of


Of DNA. This is what

Causes siblings of

The same parents to look

Similar, but not


12.2A: Genes as the Unit of Heredity - Biology

Following terms are used in gene regulation:

1) Split genes :The non-essential or non-coding parts intermixed with essential or coding parts are called split genes. The coding parts or genes are called exons and non-coding or non-essential parts or genes are called introns.

2 ) Splicing :Splicing is the process in which introns are removed and exons are joined together.


3) Constitutive genes (Housekeeping genes) :The genes which are constantly expressing themselves in a cell are called as constitutive genes. Expressions of these genes are not controlled by any factors. These genes always express the character for normal cellular activities. Eg- genes for glycolysis, They are required all the time in a cell.

4) Non-constitutive genes (Luxury genes) :These are the genes which are not always expressing themselves in a cell. They are switched on or off according to the requirement of cellular activities. Eg- Lactose system inE.coli.Non-constitutive genes are of two types: Inducible and repressible.

A) Inducible genes and Inducer -All the genes present on the chromosome are not expressed simultaneously. The genes that get activated when a certain substrate (i:e an inducer) is present in the medium are called inducible genes. The phenomenon of the action of these genes is called enzyme induction and the substrate is called inducer.

For example,E.coligrown in a medium without lactose doesn't produce enzymes required for lactose metabolism. But when the same bacteria is placed in a lactose supplemented medium, it starts producing enzymes like &beta-galactosidase required for converting lactose to glucose and galactose. Therefore, since lactose is used to induce this enzyme, it is called inducer and this phenomenon is called as enzyme induction.

B) Repressible genes and repression-The genes which get inactivated due to the presence of some chemicals are called repressible genes. When E. coliids supplied with certain metabolite more than required, the action of some genes, responsible for the formation of some specific enzymes, can be inhibited or repressed. Repression may take place in the case even if the metabolite is being provided from an outer source. As a result, certain genes are repressed and do not produce enzymes. Such inactivated genes are known as repressible genes and the phenomenon is called enzyme repression.

5) Co-repressor :Molecules that binds with the repressor protein to form a functional repressor complex is called co-repressor. In a tryptophan operon, tryptophan acts as a co-repressor by binding with the repressor protein to form a complex which on binding to the promoter switches it off and hence no transcription takes place.

6) Pseudogenes :These are the genes which have homology to functional genes but are unable to produce functional products.

7) Structural genes :These genes code for substances that contribute to the specific morphological or functional trait of the cell. These genes are also known as cistrons. The genes that contain the information to determine the sequence of amino acids are called as structural genes. Such genes have structural functions coding for the proteins needed by the cells.

8) Regulatory gene :The gene codes for the product that regulates the level of expression of the structural gene. Although it is located at the site away from the structural genes, it is the key element of an operon. These genes code for special proteins known as repressor proteins that regulate transcription.

9) Promoter gene :Gene that forms the binding site of RNA polymerase for transcription is called promoter gene. Each gene may be regulated by a specific promoter.

10) Operator gene :The gene, that operates the activity of structural genes, is called operator gene. It lies adjacent to the promoter site. Structural genes are expressed or not expressed depending upon whether the operator gene is 'on' or 'off'.

11) Transpogenesor transposons (Jumping genes) :They are the segment of DNA that can move or jump from one place to another place. These types of genes cause mutations through insertions, deletion, and translocation. Barbara McClintok discovered jumping genes. She is called lady Mendel because she was the first lady geneticist.

12) Apo- repressor :It is a protein produced by the regulatory gene for blocking the working of operator gene in the presence of co-repressor.

13) Co-repressor :It is non-protein or component of repressor and product of reactions catalysed by enzymes. In tryptophan operon, the tryptophan functions as co-repressor.

14) Operon :An operon is a coordinated genetic unit consists of an operator, a promoter, and one or more structural genes whose activity is influenced by a regulator.

Operon model

Operon concept:

An operon is a coordinated genetic unit consists of an operator, a promoter, and one or more structural genes whose activity is influenced by a regulator. Jacob and Monad proposed Operon concept in 1961 in the prokaryote. There are many operons but the Lactose or Lac Operon and Tryptophan Operon are important ones.

A) Lactose (Lac) operon concept:

Jacob and Monad studied the lactose metabolism in E.coli.The bacterium when grown in a medium containing lactose, produces three enzymes namely&beta-galactosidase, galactoside permease, and thiogalactoside transacetylase. These enzymes help to break lactose into glucose and galactose.

Lactose (+ the three enzymes)&rarr Glucose + galactose.

The structure of lac operon:

The structure of lactose operon consists of :

1) Structural genes - The lac operon consists three structural genes named Z,Y, and,A codes for three enzymes &beta-galactosidase, galactoside permease, and thiogalactoside transacetylase respectively. These genes are located in a row adjacent to each other and are known as polycistrons,

2) Operator gene - A single operator gene regulates all the three structural genes. The operator gene acts as a switch.

3) Promoter gene - A single promoter gene directs proper initiation of transcription in structural genes.

4) Regulatory gene - It is located away from the structural genes and is known as inhibitor gene. The regulatory gene constantly transcribes mRNA to produce repressor protein. It is the key element of operon because the function of the operon is dependent on it.

A)When E.coli is grown in a medium in absence of lactose:

When E.coli is grown in a medium in the absence of lactose, the regulator gene produces a repressor protein that binds the operator gene and blocks its activity. RNA polymerase can bot move from promoter to structural genes. It stops the transcription of mRNA from structural genes and thus protein synthesis is switched off. Hence, no enzymes are produced.

When E.coli is grown in a medium in absence of lactose

B)When E.coli is grown in a medium in the presence of lactose:

When the lactose is introduced in the medium, lactose binds to the repressor protein. In this way, repressor protein fails to bind to the operator gene. Then the operator gene remains active and the switch is turned on. Then, RNA polymerase moves from promoter gene to the structural genes through operator gene. These structural genes Z,Y, and A genes show transcription in the presence of RNA polymerase and show transcription process. Then three enzymes are formed.

When E.coli is grown in a medium in the presence of lactose

Synthesis of enzymes is continued unless and until all lactose molecules are consumed.

Tryptophan Operon:

Tryptophan operon is a segment of DNA that regulates the synthesis of protein.The formation of protein, in this case, is controlled by five structural genes: A,B,C,D, and E.

The presence or absence of tryptophan switches off or on the transcription mRNA and protein synthesis,

1)When E.coli is grown in a medium having tryptophan:

The regulatory gene produces repressor protein known as apo-repressor binds with tryptophan to form a repressor-co-repressor complex. This functional repressor protein binds with operator gene. The operator gene is switched off. RNA polymerase cannot be transferred to structural genes. So, there is no formation of any enzymes for expression of character.

When E.coli is grown in a medium having tryptophan

2) When E.coli is grown in a medium without tryptophan:

The regulatory gene produces repressor protein known as apo-repressor. The protein cannot bind to the operator gene. Hence, the operator gene is switched on. The RNA polymerase moves forward and structural genes produce five enzymes which help in the formation of tryptophan amino acid.

When E.coli is grown in a medium without tryptophan

Thus operon model can be completed.

Keshari, Arvind K. and Kamal K. Adhikari. A Text Book of Higher Secondary Biology(Class XII). 1st. Kathmandu: Vidyarthi Pustak Bhandar, 2015.

What is a gene? the different forms of a trait a segment of DNA that is the basic unit of heredity the appearance or characteristics that are seen in an organism an organism's allele combination What controls traits and inheritance?

Genes rule your traits and what you inheret from you parents.

Gregor Mendel discovered in his experiments that a factor determined the physical characteristics of an organism. This factor is called a gene. A gene is the molecular unit of heredity in a living organism.

Genes are segments of DNA that holds the information needed to build and maintain an organism and pass genetic traits to offspring. The trait of an organism, which refers to the physical characteristics of an organism, are passed from parents to offsprings via genes. The genes also contain variant forms called ALLELE that determines which trait an organism will possess.

I think the answer for this one might be the nucleic acids

Nucleic acids control traits and inheritance.

The nucleic acid is a natural chemical compound that is capable of breaking down them to produce organic bases such as pyrimidines and purine, sugars, phosphoric acid. The major cell of a body because they carry information to other cells. They help in protein synthesis. By this, they conclude the inherited characteristic of an individual. The two major types of nucleic acid are ribonucleic acid and deoxyribonucleic acid. They make up the DNA and by this they control the inheritance and traits in an individual.

A recessive trait is defined as a trait that gets expressed by an individual when they show a homozygous recessive condition. This means that if an individual showing a recessive trait will definitely possess both the recessive alleles for that trait as a recessive trait gets concealed by the occurrence of allele for the dominant trait.

Learn more about graffain follicle Learn more about human sperm and egg cells Learn more about female reproductive tract

Nucleic acid, individual, trait, recessive, dominant, organic, sugar, purine, pyrimidines, cell, phosphoric acid, allele, ribonucleic acid, deoxyribonucleic acid.

Genetics: The Study of Heredity

Read this article to learn about the genetics:- the study of heredity. It is appropriately regarded as the science that explains the similarities and differences among the related organisms.

The Blood Theory of Inheritance in Humans:

For many centuries, it was customary to explain inheritance in humans through blood theory. People used to believe that the children received blood from their parents, and it was the union of blood that led to the blending of characteristics.

That is how the terms ‘blood relations’, ‘blood will tell’, and ‘blood is thicker than water’ came into existence. They are still used, despite the fact that blood is no more involved in inheritance.

With the advances in genetics, the more appropriate terms should be as follows:

I. Gene relations in place of blood relations.

II. Genes will tell instead of blood will tell.

Brief History and Development of Genetics:

Genetics is relatively young, not even 150 years. The blood theory of inheritance was questioned in 1850s, based on the fact that the semen contained no blood. Thus, blood was not being transferred to the offspring. Then the big question was what was the hereditary substance.

It was in 1866, an Austrian monk named Gregor Johann Mendel, for the first time reported the fundamental laws of inheritance. He conducted several experiments on the breeding patterns of pea plants. Mendel put forth the theory of transmissible factors which states that inheritance is controlled by certain factors passed from parents to offspring’s. His results were published in 1866 in an obscure journal Proceedings of the Society of Natural Sciences.

For about 35 years, the observations made by Mendel went unnoticed, and were almost forgotten. Two European botanists (Correns and Hugo de Vries) in 1900, independently and simultaneously rediscovered the theories of Mendel. The year 1900 is important as it marks the beginning the modern era of genetics.

The origin of the word gene:

In the early years of twentieth century, it was believed that the Mendel’s inheritance factors are very closely related to chromosomes (literally coloured bodies) of the cells. It was in 1920s, the term gene (derived from a Greek word gennan meaning to produce) was introduced by Willard Johannsen. Thus, gene replaced the earlier terms inheritance factor or inheritance unit.

Chemical basis of heredity:

There was a controversy for quite some time on the chemical basis of inheritance. There were two groups—the protein supporters and DNA supporters. It was in 1944, Avery and his associates presented convincing evidence that the chemical basis of heredity lies in DNA, and not in protein. Thus, DMA was finally identified as the genetic material. Its structure was elucidated in 1952 by Watson and Crick.

Basic Principles of Heredity in Humans:

The understanding of how genetic characteristics are passed on from one generation to the next is based on the principles developed by Mendel. As we know now, the human genome is organized into a diploid (2n) set of 46 chromosomes. They exist as 22 pairs of autosomes and one pair of sex chromosomes (XX/XY). During the course of meiosis, the chromosome number becomes haploid (n). Thus, haploid male and female gametes — sperm and oocyte respectively, are formed.

On fertilization of the oocyte by the sperm, the diploid status is restored. This becomes possible as the zygote receives one member of each chromosome pair from the father, and the other from the mother. As regards the sex chromosomes, the males have X and Y, while the females have XX. The sex of the child is determined by the father.

Monogenic and Polygenic Traits:

The genetic traits or characters are controlled by single genes or multiple genes. The changes in genes are associated with genetic diseases.

These are the single gene disease traits due to alterations in the corresponding gene e.g. Sickle-cell anemia, phenylketonuria. Inheritance of monogenic disorders usually follows the Mendelian pattern of inheritance.

The genetic traits conferred by more than on gene (i.e multiple genes), and the disorders associated with them are very important e.g. height, weight, skin colours, academic performance, blood pressure, aggressiveness, length of life.

Patterns of Inheritance:

The heredity is transmitted from parent to offspring as individual characters controlled by genes. The genes are linearly distributed on chromosomes at fixed positions called loci. A gene may have different forms referred to as alleles. Usually one allele is transferred from the father, and the other from the mother.

The allele is regarded as dominant if the trait is exhibited due to its presence. On the other hand, the allele is said to be recessive if its effect is masked by a dominant allele. The individuals are said to be homozygous if both the alleles are the same. When the alleles are different they are said to be heterozygous.

The pattern of inheritance of monogenic traits may occur in the following ways (Fig. 69.1).

1. Autosomal dominant inheritance:

A normal allele may be designated as a while an autosomal dominant disease allele as A (Fig. 69.1 A). The male with Aa genotype is an affected one while the female with aa is normal. Half of the genes from the affected male will carry the disease allele.

On mating, the male and female gametes are mixed in different combinations. The result is that half of the children will be heterozygous (Aa) and have the disease. Example of autosomal dominant inherited diseases are familial hypercholesterolemia, β-thalassemia, breast cancer genes.

2. Autosomal recessive inheritance:

In this case, the normal allele is designated as B while the disease-causing one is a (Fig. 69.1 B). The gametes of carrier male and carrier female (both with genotype Bb) get mixed. For these heterozygous carrier parents, there is one fourth chance of having an affected child. Cystic fibrosis, sickle-cell anemia and phenylketonuria are some good examples of autosomal recessive disorders.

3. Sex (X)-linked inheritance:

In the Fig. 69.1 C, sex-linked pattern of inheritance is depicted. A normal male (XY) and a carrier female (X C Y) will produce children wherein, half of the male children are affected while no female children is affected. This is due to the fact that the male children possess only one X chromosome, and there is no dominant allele to mark its effects (as is the case with females). Colour blindness and hemophilia are good examples of X-linked diseases.

A selected list of genetic disorders (monogenic traits) due to autosomal and sex-linked inheritance in humans is given in Table 69.1.

Genetic Diseases in Humans:

The pattern of inheritance and monogenic traits along with some of the associated disorders are described above (Table 69.1). Besides gene mutations, chromosomal abnormalities (aberrations) also result in genetic diseases.

The presence of abnormal number of chromosomes within the cells is referred to as aneuploidy. The most common aneuploid condition is trisomy in which three copies of a particular chromosome are present in a cell instead of the normal two e.g. trisomy-21 causing Down’s syndrome-, trisomy-18 that results in Edward’s syndrome. These are the examples of autosomal aneuploidy. In case of sex-linked aneuploidy, the sex chromosomes occur as three copies, e.g. phenotypically male causing Klinefelter’s syndrome has XXY trisomy-X is phenotically a female with XXX.

Selected examples of chromosomal disorders along the with the syndromes and their characteristic features are given in Table 69.2.


Eugenics is a science of improving human race based on genetics. Improving the traits of plants and animals through breeding programmes has been in practice for centuries. Eugenics is a highly controversial subject due to social, ethical, and political reasons. The proponents of eugenics argue that people with desirable and good traits (good blood) should reproduce while those with undesirable characters (bad blood) should not.

The advocates of eugenics, however, do not force any policy, but they try to convince the people to perform their duty voluntarily. The object of eugenics is to limit the production of people who are unfit to live in the society.

Eugenics in Nazi Germany:

Germany developed its own eugenic programme during 1930s. A law on eugenic sterilization was passed in 1933. In a span of three years, compulsory sterilization was done on about 250,000 people, who allegedly suffered from hereditary disabilities, feeble mindedness, epilepsy, schizophrenia, blindness, physical deformities, and drug or alcohol addition. The German Government committed many atrocities in the name of racial purity. Other countries however do not support this kind of eugenics.


Discovery of discrete inherited units Edit

The existence of discrete inheritable units was first suggested by Gregor Mendel (1822–1884). [11] From 1857 to 1864, in Brno, Austrian Empire (today's Czech Republic), he studied inheritance patterns in 8000 common edible pea plants, tracking distinct traits from parent to offspring. He described these mathematically as 2 n combinations where n is the number of differing characteristics in the original peas. Although he did not use the term gene, he explained his results in terms of discrete inherited units that give rise to observable physical characteristics. This description prefigured Wilhelm Johannsen's distinction between genotype (the genetic material of an organism) and phenotype (the observable traits of that organism). Mendel was also the first to demonstrate independent assortment, the distinction between dominant and recessive traits, the distinction between a heterozygote and homozygote, and the phenomenon of discontinuous inheritance.

Prior to Mendel's work, the dominant theory of heredity was one of blending inheritance, which suggested that each parent contributed fluids to the fertilisation process and that the traits of the parents blended and mixed to produce the offspring. Charles Darwin developed a theory of inheritance he termed pangenesis, from Greek pan ("all, whole") and genesis ("birth") / genos ("origin"). [12] [13] Darwin used the term gemmule to describe hypothetical particles that would mix during reproduction.

Mendel's work went largely unnoticed after its first publication in 1866, but was rediscovered in the late 19th century by Hugo de Vries, Carl Correns, and Erich von Tschermak, who (claimed to have) reached similar conclusions in their own research. [14] Specifically, in 1889, Hugo de Vries published his book Intracellular Pangenesis, [15] in which he postulated that different characters have individual hereditary carriers and that inheritance of specific traits in organisms comes in particles. De Vries called these units "pangenes" (Pangens in German), after Darwin's 1868 pangenesis theory.

Twenty years later, in 1909, Wilhelm Johannsen introduced the term 'gene' [10] and in 1906, William Bateson, that of 'genetics' [16] [17] while Eduard Strasburger, amongst others, still used the term 'pangene' for the fundamental physical and functional unit of heredity. [15] : Translator's preface, viii

Discovery of DNA Edit

Advances in understanding genes and inheritance continued throughout the 20th century. Deoxyribonucleic acid (DNA) was shown to be the molecular repository of genetic information by experiments in the 1940s to 1950s. [18] [19] The structure of DNA was studied by Rosalind Franklin and Maurice Wilkins using X-ray crystallography, which led James D. Watson and Francis Crick to publish a model of the double-stranded DNA molecule whose paired nucleotide bases indicated a compelling hypothesis for the mechanism of genetic replication. [20] [21]

In the early 1950s the prevailing view was that the genes in a chromosome acted like discrete entities, indivisible by recombination and arranged like beads on a string. The experiments of Benzer using mutants defective in the rII region of bacteriophage T4 (1955–1959) showed that individual genes have a simple linear structure and are likely to be equivalent to a linear section of DNA. [22] [23]

Collectively, this body of research established the central dogma of molecular biology, which states that proteins are translated from RNA, which is transcribed from DNA. This dogma has since been shown to have exceptions, such as reverse transcription in retroviruses. The modern study of genetics at the level of DNA is known as molecular genetics.

In 1972, Walter Fiers and his team were the first to determine the sequence of a gene: that of Bacteriophage MS2 coat protein. [24] The subsequent development of chain-termination DNA sequencing in 1977 by Frederick Sanger improved the efficiency of sequencing and turned it into a routine laboratory tool. [25] An automated version of the Sanger method was used in early phases of the Human Genome Project. [26]

Modern synthesis and its successors Edit

The theories developed in the early 20th century to integrate Mendelian genetics with Darwinian evolution are called the modern synthesis, a term introduced by Julian Huxley. [27]

Evolutionary biologists have subsequently modified this concept, such as George C. Williams' gene-centric view of evolution. He proposed an evolutionary concept of the gene as a unit of natural selection with the definition: "that which segregates and recombines with appreciable frequency." [28] : 24 In this view, the molecular gene transcribes as a unit, and the evolutionary gene inherits as a unit. Related ideas emphasizing the centrality of genes in evolution were popularized by Richard Dawkins. [29] [30]

DNA Edit

The vast majority of organisms encode their genes in long strands of DNA (deoxyribonucleic acid). DNA consists of a chain made from four types of nucleotide subunits, each composed of: a five-carbon sugar (2-deoxyribose), a phosphate group, and one of the four bases adenine, cytosine, guanine, and thymine. [31] : 2.1

Two chains of DNA twist around each other to form a DNA double helix with the phosphate-sugar backbone spiraling around the outside, and the bases pointing inwards with adenine base pairing to thymine and guanine to cytosine. The specificity of base pairing occurs because adenine and thymine align to form two hydrogen bonds, whereas cytosine and guanine form three hydrogen bonds. The two strands in a double helix must, therefore, be complementary, with their sequence of bases matching such that the adenines of one strand are paired with the thymines of the other strand, and so on. [31] : 4.1

Due to the chemical composition of the pentose residues of the bases, DNA strands have directionality. One end of a DNA polymer contains an exposed hydroxyl group on the deoxyribose this is known as the 3' end of the molecule. The other end contains an exposed phosphate group this is the 5' end. The two strands of a double-helix run in opposite directions. Nucleic acid synthesis, including DNA replication and transcription occurs in the 5'→3' direction, because new nucleotides are added via a dehydration reaction that uses the exposed 3' hydroxyl as a nucleophile. [32] : 27.2

The expression of genes encoded in DNA begins by transcribing the gene into RNA, a second type of nucleic acid that is very similar to DNA, but whose monomers contain the sugar ribose rather than deoxyribose. RNA also contains the base uracil in place of thymine. RNA molecules are less stable than DNA and are typically single-stranded. Genes that encode proteins are composed of a series of three-nucleotide sequences called codons, which serve as the "words" in the genetic "language". The genetic code specifies the correspondence during protein translation between codons and amino acids. The genetic code is nearly the same for all known organisms. [31] : 4.1

Chromosomes Edit

The total complement of genes in an organism or cell is known as its genome, which may be stored on one or more chromosomes. A chromosome consists of a single, very long DNA helix on which thousands of genes are encoded. [31] : 4.2 The region of the chromosome at which a particular gene is located is called its locus. Each locus contains one allele of a gene however, members of a population may have different alleles at the locus, each with a slightly different gene sequence.

The majority of eukaryotic genes are stored on a set of large, linear chromosomes. The chromosomes are packed within the nucleus in complex with storage proteins called histones to form a unit called a nucleosome. DNA packaged and condensed in this way is called chromatin. [31] : 4.2 The manner in which DNA is stored on the histones, as well as chemical modifications of the histone itself, regulate whether a particular region of DNA is accessible for gene expression. In addition to genes, eukaryotic chromosomes contain sequences involved in ensuring that the DNA is copied without degradation of end regions and sorted into daughter cells during cell division: replication origins, telomeres and the centromere. [31] : 4.2 Replication origins are the sequence regions where DNA replication is initiated to make two copies of the chromosome. Telomeres are long stretches of repetitive sequences that cap the ends of the linear chromosomes and prevent degradation of coding and regulatory regions during DNA replication. The length of the telomeres decreases each time the genome is replicated and has been implicated in the aging process. [34] The centromere is required for binding spindle fibres to separate sister chromatids into daughter cells during cell division. [31] : 18.2

Prokaryotes (bacteria and archaea) typically store their genomes on a single large, circular chromosome. Similarly, some eukaryotic organelles contain a remnant circular chromosome with a small number of genes. [31] : 14.4 Prokaryotes sometimes supplement their chromosome with additional small circles of DNA called plasmids, which usually encode only a few genes and are transferable between individuals. For example, the genes for antibiotic resistance are usually encoded on bacterial plasmids and can be passed between individual cells, even those of different species, via horizontal gene transfer. [35]

Whereas the chromosomes of prokaryotes are relatively gene-dense, those of eukaryotes often contain regions of DNA that serve no obvious function. Simple single-celled eukaryotes have relatively small amounts of such DNA, whereas the genomes of complex multicellular organisms, including humans, contain an absolute majority of DNA without an identified function. [36] This DNA has often been referred to as "junk DNA". However, more recent analyses suggest that, although protein-coding DNA makes up barely 2% of the human genome, about 80% of the bases in the genome may be expressed, so the term "junk DNA" may be a misnomer. [9]

Structure Edit

The structure of a gene consists of many elements of which the actual protein coding sequence is often only a small part. These include DNA regions that are not transcribed as well as untranslated regions of the RNA.

Flanking the open reading frame, genes contain a regulatory sequence that is required for their expression. First, genes require a promoter sequence. The promoter is recognized and bound by transcription factors that recruit and help RNA polymerase bind to the region to initiate transcription. [31] : 7.1 The recognition typically occurs as a consensus sequence like the TATA box. A gene can have more than one promoter, resulting in messenger RNAs (mRNA) that differ in how far they extend in the 5' end. [38] Highly transcribed genes have "strong" promoter sequences that form strong associations with transcription factors, thereby initiating transcription at a high rate. Others genes have "weak" promoters that form weak associations with transcription factors and initiate transcription less frequently. [31] : 7.2 Eukaryotic promoter regions are much more complex and difficult to identify than prokaryotic promoters. [31] : 7.3

Additionally, genes can have regulatory regions many kilobases upstream or downstream of the open reading frame that alter expression. These act by binding to transcription factors which then cause the DNA to loop so that the regulatory sequence (and bound transcription factor) become close to the RNA polymerase binding site. [39] For example, enhancers increase transcription by binding an activator protein which then helps to recruit the RNA polymerase to the promoter conversely silencers bind repressor proteins and make the DNA less available for RNA polymerase. [40]

The transcribed pre-mRNA contains untranslated regions at both ends which contain a ribosome binding site, terminator and start and stop codons. [41] In addition, most eukaryotic open reading frames contain untranslated introns which are removed before the exons are translated. The sequences at the ends of the introns dictate the splice sites to generate the final mature mRNA which encodes the protein or RNA product. [42]

Many prokaryotic genes are organized into operons, with multiple protein-coding sequences that are transcribed as a unit. [43] [44] The genes in an operon are transcribed as a continuous messenger RNA, referred to as a polycistronic mRNA. The term cistron in this context is equivalent to gene. The transcription of an operon's mRNA is often controlled by a repressor that can occur in an active or inactive state depending on the presence of specific metabolites. [45] When active, the repressor binds to a DNA sequence at the beginning of the operon, called the operator region, and represses transcription of the operon when the repressor is inactive transcription of the operon can occur (see e.g. Lac operon). The products of operon genes typically have related functions and are involved in the same regulatory network. [31] : 7.3

Functional definitions Edit

Defining exactly what section of a DNA sequence comprises a gene is difficult. [7] [46] Regulatory regions of a gene such as enhancers do not necessarily have to be close to the coding sequence on the linear molecule because the intervening DNA can be looped out to bring the gene and its regulatory region into proximity. Similarly, a gene's introns can be much larger than its exons. Regulatory regions can even be on entirely different chromosomes and operate in trans to allow regulatory regions on one chromosome to come in contact with target genes on another chromosome. [47] [48]

Early work in molecular genetics suggested the concept that one gene makes one protein. This concept (originally called the one gene-one enzyme hypothesis) emerged from an influential 1941 paper by George Beadle and Edward Tatum on experiments with mutants of the fungus Neurospora crassa. [49] Norman Horowitz, an early colleague on the Neurospora research, reminisced in 2004 that “these experiments founded the science of what Beadle and Tatum called biochemical genetics. In actuality they proved to be the opening gun in what became molecular genetics and all the developments that have followed from that.” [50] The one gene-one protein concept has been refined since the discovery of genes that can encode multiple proteins by alternative splicing and coding sequences split in short section across the genome whose mRNAs are concatenated by trans-splicing. [9] [51] [52]

A broad operational definition is sometimes used to encompass the complexity of these diverse phenomena, where a gene is defined as a union of genomic sequences encoding a coherent set of potentially overlapping functional products. [17] This definition categorizes genes by their functional products (proteins or RNA) rather than their specific DNA loci, with regulatory elements classified as gene-associated regions. [17]

In all organisms, two steps are required to read the information encoded in a gene's DNA and produce the protein it specifies. First, the gene's DNA is transcribed to messenger RNA (mRNA). [31] : 6.1 Second, that mRNA is translated to protein. [31] : 6.2 RNA-coding genes must still go through the first step, but are not translated into protein. [53] The process of producing a biologically functional molecule of either RNA or protein is called gene expression, and the resulting molecule is called a gene product.

Genetic code Edit

The nucleotide sequence of a gene's DNA specifies the amino acid sequence of a protein through the genetic code. Sets of three nucleotides, known as codons, each correspond to a specific amino acid. [31] : 6 The principle that three sequential bases of DNA code for each amino acid was demonstrated in 1961 using frameshift mutations in the rIIB gene of bacteriophage T4 [54] (see Crick, Brenner et al. experiment).

Additionally, a "start codon", and three "stop codons" indicate the beginning and end of the protein coding region. There are 64 possible codons (four possible nucleotides at each of three positions, hence 4 3 possible codons) and only 20 standard amino acids hence the code is redundant and multiple codons can specify the same amino acid. The correspondence between codons and amino acids is nearly universal among all known living organisms. [55]

Transcription Edit

Transcription produces a single-stranded RNA molecule known as messenger RNA, whose nucleotide sequence is complementary to the DNA from which it was transcribed. [31] : 6.1 The mRNA acts as an intermediate between the DNA gene and its final protein product. The gene's DNA is used as a template to generate a complementary mRNA. The mRNA matches the sequence of the gene's DNA coding strand because it is synthesised as the complement of the template strand. Transcription is performed by an enzyme called an RNA polymerase, which reads the template strand in the 3' to 5' direction and synthesizes the RNA from 5' to 3'. To initiate transcription, the polymerase first recognizes and binds a promoter region of the gene. Thus, a major mechanism of gene regulation is the blocking or sequestering the promoter region, either by tight binding by repressor molecules that physically block the polymerase or by organizing the DNA so that the promoter region is not accessible. [31] : 7

In prokaryotes, transcription occurs in the cytoplasm for very long transcripts, translation may begin at the 5' end of the RNA while the 3' end is still being transcribed. In eukaryotes, transcription occurs in the nucleus, where the cell's DNA is stored. The RNA molecule produced by the polymerase is known as the primary transcript and undergoes post-transcriptional modifications before being exported to the cytoplasm for translation. One of the modifications performed is the splicing of introns which are sequences in the transcribed region that do not encode a protein. Alternative splicing mechanisms can result in mature transcripts from the same gene having different sequences and thus coding for different proteins. This is a major form of regulation in eukaryotic cells and also occurs in some prokaryotes. [31] : 7.5 [56]

Translation Edit

Translation is the process by which a mature mRNA molecule is used as a template for synthesizing a new protein. [31] : 6.2 Translation is carried out by ribosomes, large complexes of RNA and protein responsible for carrying out the chemical reactions to add new amino acids to a growing polypeptide chain by the formation of peptide bonds. The genetic code is read three nucleotides at a time, in units called codons, via interactions with specialized RNA molecules called transfer RNA (tRNA). Each tRNA has three unpaired bases known as the anticodon that are complementary to the codon it reads on the mRNA. The tRNA is also covalently attached to the amino acid specified by the complementary codon. When the tRNA binds to its complementary codon in an mRNA strand, the ribosome attaches its amino acid cargo to the new polypeptide chain, which is synthesized from amino terminus to carboxyl terminus. During and after synthesis, most new proteins must fold to their active three-dimensional structure before they can carry out their cellular functions. [31] : 3

Regulation Edit

Genes are regulated so that they are expressed only when the product is needed, since expression draws on limited resources. [31] : 7 A cell regulates its gene expression depending on its external environment (e.g. available nutrients, temperature and other stresses), its internal environment (e.g. cell division cycle, metabolism, infection status), and its specific role if in a multicellular organism. Gene expression can be regulated at any step: from transcriptional initiation, to RNA processing, to post-translational modification of the protein. The regulation of lactose metabolism genes in E. coli (lac operon) was the first such mechanism to be described in 1961. [57]

RNA genes Edit

A typical protein-coding gene is first copied into RNA as an intermediate in the manufacture of the final protein product. [31] : 6.1 In other cases, the RNA molecules are the actual functional products, as in the synthesis of ribosomal RNA and transfer RNA. Some RNAs known as ribozymes are capable of enzymatic function, and microRNA has a regulatory role. The DNA sequences from which such RNAs are transcribed are known as non-coding RNA genes. [53]

Some viruses store their entire genomes in the form of RNA, and contain no DNA at all. [58] [59] Because they use RNA to store genes, their cellular hosts may synthesize their proteins as soon as they are infected and without the delay in waiting for transcription. [60] On the other hand, RNA retroviruses, such as HIV, require the reverse transcription of their genome from RNA into DNA before their proteins can be synthesized. RNA-mediated epigenetic inheritance has also been observed in plants and very rarely in animals. [61]

Organisms inherit their genes from their parents. Asexual organisms simply inherit a complete copy of their parent's genome. Sexual organisms have two copies of each chromosome because they inherit one complete set from each parent. [31] : 1

Mendelian inheritance Edit

According to Mendelian inheritance, variations in an organism's phenotype (observable physical and behavioral characteristics) are due in part to variations in its genotype (particular set of genes). Each gene specifies a particular trait with a different sequence of a gene (alleles) giving rise to different phenotypes. Most eukaryotic organisms (such as the pea plants Mendel worked on) have two alleles for each trait, one inherited from each parent. [31] : 20

Alleles at a locus may be dominant or recessive dominant alleles give rise to their corresponding phenotypes when paired with any other allele for the same trait, whereas recessive alleles give rise to their corresponding phenotype only when paired with another copy of the same allele. If you know the genotypes of the organisms, you can determine which alleles are dominant and which are recessive. For example, if the allele specifying tall stems in pea plants is dominant over the allele specifying short stems, then pea plants that inherit one tall allele from one parent and one short allele from the other parent will also have tall stems. Mendel's work demonstrated that alleles assort independently in the production of gametes, or germ cells, ensuring variation in the next generation. Although Mendelian inheritance remains a good model for many traits determined by single genes (including a number of well-known genetic disorders) it does not include the physical processes of DNA replication and cell division. [62] [63]

DNA replication and cell division Edit

The growth, development, and reproduction of organisms relies on cell division the process by which a single cell divides into two usually identical daughter cells. This requires first making a duplicate copy of every gene in the genome in a process called DNA replication. [31] : 5.2 The copies are made by specialized enzymes known as DNA polymerases, which "read" one strand of the double-helical DNA, known as the template strand, and synthesize a new complementary strand. Because the DNA double helix is held together by base pairing, the sequence of one strand completely specifies the sequence of its complement hence only one strand needs to be read by the enzyme to produce a faithful copy. The process of DNA replication is semiconservative that is, the copy of the genome inherited by each daughter cell contains one original and one newly synthesized strand of DNA. [31] : 5.2

The rate of DNA replication in living cells was first measured as the rate of phage T4 DNA elongation in phage-infected E. coli and found to be impressively rapid. [64] During the period of exponential DNA increase at 37 °C, the rate of elongation was 749 nucleotides per second.

After DNA replication is complete, the cell must physically separate the two copies of the genome and divide into two distinct membrane-bound cells. [31] : 18.2 In prokaryotes (bacteria and archaea) this usually occurs via a relatively simple process called binary fission, in which each circular genome attaches to the cell membrane and is separated into the daughter cells as the membrane invaginates to split the cytoplasm into two membrane-bound portions. Binary fission is extremely fast compared to the rates of cell division in eukaryotes. Eukaryotic cell division is a more complex process known as the cell cycle DNA replication occurs during a phase of this cycle known as S phase, whereas the process of segregating chromosomes and splitting the cytoplasm occurs during M phase. [31] : 18.1

Molecular inheritance Edit

The duplication and transmission of genetic material from one generation of cells to the next is the basis for molecular inheritance and the link between the classical and molecular pictures of genes. Organisms inherit the characteristics of their parents because the cells of the offspring contain copies of the genes in their parents' cells. In asexually reproducing organisms, the offspring will be a genetic copy or clone of the parent organism. In sexually reproducing organisms, a specialized form of cell division called meiosis produces cells called gametes or germ cells that are haploid, or contain only one copy of each gene. [31] : 20.2 The gametes produced by females are called eggs or ova, and those produced by males are called sperm. Two gametes fuse to form a diploid fertilized egg, a single cell that has two sets of genes, with one copy of each gene from the mother and one from the father. [31] : 20

During the process of meiotic cell division, an event called genetic recombination or crossing-over can sometimes occur, in which a length of DNA on one chromatid is swapped with a length of DNA on the corresponding homologous non-sister chromatid. This can result in reassortment of otherwise linked alleles. [31] : 5.5 The Mendelian principle of independent assortment asserts that each of a parent's two genes for each trait will sort independently into gametes which allele an organism inherits for one trait is unrelated to which allele it inherits for another trait. This is in fact only true for genes that do not reside on the same chromosome or are located very far from one another on the same chromosome. The closer two genes lie on the same chromosome, the more closely they will be associated in gametes and the more often they will appear together (known as genetic linkage). [65] Genes that are very close are essentially never separated because it is extremely unlikely that a crossover point will occur between them. [65]

Mutation Edit

DNA replication is for the most part extremely accurate, however errors (mutations) do occur. [31] : 7.6 The error rate in eukaryotic cells can be as low as 10 −8 per nucleotide per replication, [66] [67] whereas for some RNA viruses it can be as high as 10 −3 . [68] This means that each generation, each human genome accumulates 1–2 new mutations. [68] Small mutations can be caused by DNA replication and the aftermath of DNA damage and include point mutations in which a single base is altered and frameshift mutations in which a single base is inserted or deleted. Either of these mutations can change the gene by missense (change a codon to encode a different amino acid) or nonsense (a premature stop codon). [69] Larger mutations can be caused by errors in recombination to cause chromosomal abnormalities including the duplication, deletion, rearrangement or inversion of large sections of a chromosome. Additionally, DNA repair mechanisms can introduce mutational errors when repairing physical damage to the molecule. The repair, even with mutation, is more important to survival than restoring an exact copy, for example when repairing double-strand breaks. [31] : 5.4

When multiple different alleles for a gene are present in a species's population it is called polymorphic. Most different alleles are functionally equivalent, however some alleles can give rise to different phenotypic traits. A gene's most common allele is called the wild type, and rare alleles are called mutants. The genetic variation in relative frequencies of different alleles in a population is due to both natural selection and genetic drift. [70] The wild-type allele is not necessarily the ancestor of less common alleles, nor is it necessarily fitter.

Most mutations within genes are neutral, having no effect on the organism's phenotype (silent mutations). Some mutations do not change the amino acid sequence because multiple codons encode the same amino acid (synonymous mutations). Other mutations can be neutral if they lead to amino acid sequence changes, but the protein still functions similarly with the new amino acid (e.g. conservative mutations). Many mutations, however, are deleterious or even lethal, and are removed from populations by natural selection. Genetic disorders are the result of deleterious mutations and can be due to spontaneous mutation in the affected individual, or can be inherited. Finally, a small fraction of mutations are beneficial, improving the organism's fitness and are extremely important for evolution, since their directional selection leads to adaptive evolution. [31] : 7.6

Sequence homology Edit

Genes with a most recent common ancestor, and thus a shared evolutionary ancestry, are known as homologs. [71] These genes appear either from gene duplication within an organism's genome, where they are known as paralogous genes, or are the result of divergence of the genes after a speciation event, where they are known as orthologous genes, [31] : 7.6 and often perform the same or similar functions in related organisms. It is often assumed that the functions of orthologous genes are more similar than those of paralogous genes, although the difference is minimal. [72] [73]

The relationship between genes can be measured by comparing the sequence alignment of their DNA. [31] : 7.6 The degree of sequence similarity between homologous genes is called conserved sequence. Most changes to a gene's sequence do not affect its function and so genes accumulate mutations over time by neutral molecular evolution. Additionally, any selection on a gene will cause its sequence to diverge at a different rate. Genes under stabilizing selection are constrained and so change more slowly whereas genes under directional selection change sequence more rapidly. [74] The sequence differences between genes can be used for phylogenetic analyses to study how those genes have evolved and how the organisms they come from are related. [75] [76]

Origins of new genes Edit

The most common source of new genes in eukaryotic lineages is gene duplication, which creates copy number variation of an existing gene in the genome. [77] [78] The resulting genes (paralogs) may then diverge in sequence and in function. Sets of genes formed in this way compose a gene family. Gene duplications and losses within a family are common and represent a major source of evolutionary biodiversity. [79] Sometimes, gene duplication may result in a nonfunctional copy of a gene, or a functional copy may be subject to mutations that result in loss of function such nonfunctional genes are called pseudogenes. [31] : 7.6

"Orphan" genes, whose sequence shows no similarity to existing genes, are less common than gene duplicates. The human genome contains an estimate 18 [80] to 60 [81] genes with no identifiable homologs outside humans. Orphan genes arise primarily from either de novo emergence from previously non-coding sequence, or gene duplication followed by such rapid sequence change that the original relationship becomes undetectable. [82] De novo genes are typically shorter and simpler in structure than most eukaryotic genes, with few if any introns. [77] Over long evolutionary time periods, de novo gene birth may be responsible for a significant fraction of taxonomically-restricted gene families. [83]

Horizontal gene transfer refers to the transfer of genetic material through a mechanism other than reproduction. This mechanism is a common source of new genes in prokaryotes, sometimes thought to contribute more to genetic variation than gene duplication. [84] It is a common means of spreading antibiotic resistance, virulence, and adaptive metabolic functions. [35] [85] Although horizontal gene transfer is rare in eukaryotes, likely examples have been identified of protist and alga genomes containing genes of bacterial origin. [86] [87]

The genome is the total genetic material of an organism and includes both the genes and non-coding sequences. [88] Eukaryotic genes can be annotated using FINDER. [89]

Number of genes Edit

The genome size, and the number of genes it encodes varies widely between organisms. The smallest genomes occur in viruses, [98] and viroids (which act as a single non-coding RNA gene). [99] Conversely, plants can have extremely large genomes, [100] with rice containing >46,000 protein-coding genes. [94] The total number of protein-coding genes (the Earth's proteome) is estimated to be 5 million sequences. [101]

Although the number of base-pairs of DNA in the human genome has been known since the 1960s, the estimated number of genes has changed over time as definitions of genes, and methods of detecting them have been refined. Initial theoretical predictions of the number of human genes were as high as 2,000,000. [102] Early experimental measures indicated there to be 50,000–100,000 transcribed genes (expressed sequence tags). [103] Subsequently, the sequencing in the Human Genome Project indicated that many of these transcripts were alternative variants of the same genes, and the total number of protein-coding genes was revised down to

20,000 [97] with 13 genes encoded on the mitochondrial genome. [95] With the GENCODE annotation project, that estimate has continued to fall to 19,000. [104] Of the human genome, only 1–2% consists of protein-coding sequences, [105] with the remainder being 'noncoding' DNA such as introns, retrotransposons, and noncoding RNAs. [105] [106] Every multicellular organism has all its genes in each cell of its body but not every gene functions in every cell .

Essential genes Edit

Essential genes are the set of genes thought to be critical for an organism's survival. [108] This definition assumes the abundant availability of all relevant nutrients and the absence of environmental stress. Only a small portion of an organism's genes are essential. In bacteria, an estimated 250–400 genes are essential for Escherichia coli and Bacillus subtilis, which is less than 10% of their genes. [109] [110] [111] Half of these genes are orthologs in both organisms and are largely involved in protein synthesis. [111] In the budding yeast Saccharomyces cerevisiae the number of essential genes is slightly higher, at 1000 genes (

20% of their genes). [112] Although the number is more difficult to measure in higher eukaryotes, mice and humans are estimated to have around 2000 essential genes (

10% of their genes). [113] The synthetic organism, Syn 3, has a minimal genome of 473 essential genes and quasi-essential genes (necessary for fast growth), although 149 have unknown function. [107]

Essential genes include housekeeping genes (critical for basic cell functions) [114] as well as genes that are expressed at different times in the organisms development or life cycle. [115] Housekeeping genes are used as experimental controls when analysing gene expression, since they are constitutively expressed at a relatively constant level.

Genetic and genomic nomenclature Edit

Gene nomenclature has been established by the HUGO Gene Nomenclature Committee (HGNC), a committee of the Human Genome Organisation, for each known human gene in the form of an approved gene name and symbol (short-form abbreviation), which can be accessed through a database maintained by HGNC. Symbols are chosen to be unique, and each gene has only one symbol (although approved symbols sometimes change). Symbols are preferably kept consistent with other members of a gene family and with homologs in other species, particularly the mouse due to its role as a common model organism. [116]

Genetic engineering is the modification of an organism's genome through biotechnology. Since the 1970s, a variety of techniques have been developed to specifically add, remove and edit genes in an organism. [117] Recently developed genome engineering techniques use engineered nuclease enzymes to create targeted DNA repair in a chromosome to either disrupt or edit a gene when the break is repaired. [118] [119] [120] [121] The related term synthetic biology is sometimes used to refer to extensive genetic engineering of an organism. [122]

Genetic engineering is now a routine research tool with model organisms. For example, genes are easily added to bacteria [123] and lineages of knockout mice with a specific gene's function disrupted are used to investigate that gene's function. [124] [125] Many organisms have been genetically modified for applications in agriculture, industrial biotechnology, and medicine.

For multicellular organisms, typically the embryo is engineered which grows into the adult genetically modified organism. [126] However, the genomes of cells in an adult organism can be edited using gene therapy techniques to treat genetic diseases.

Gene Examples

The gene examples listed here are recent examples. A list composed in the future may differ. Due to the current surge in genetic research and our understanding of the codes that make each organism unique, gene examples are constantly evolving.

RNA Virus Genes

Viruses can be categorized according to gene type. They can be either RNA or DNA viruses. Genes found inside the virus are few in number, from a handful to a maximum of around 200 genes.

Viruses can also change their genetic information by way of recombination, where two viruses inside a host organism exchange their genetic material. A virus (retrovirus) can also insert a copy of its genome into host cells.

This ability to constantly change the genetic code means RNA viruses can adapt to survive and replicate in previously immune or resistant hosts. Some of the world’s most feared viruses are RNA viruses. This group of pathogens includes the viruses that cause Ebola, rabies, influenza, West Nile fever, polio, and measles (pictured below).

An example of a viral gene would be BALF5. This gene produces a DNA polymerase protein subunit in the Epstein-Barr virus.

Bacterial Genes

Bacteria are estimated to have between 500 and 7500 genes, depending on their complexity. Many bacteria have a single chromosome containing the bacterial genome, as well as separate structures called plasmids, which can replicate independently of the chromosome. This gives the DNA inside plasmids the name ‘extrachromosomal DNA’. While bacterial chromosomes are usually reported to be circular in form, they can also be linear. A basic diagram can be seen below.

An example of a bacterial gene is blaOXA-2, which encodes a protein that contributes to beta-lactamase production. The finished product is an enzyme that is known to increase the resistance of many bacteria, including Escherichia Coli, to beta-lactam antibiotics.

Human Genes

The more complex the organism, the more complex its genome and the higher the number of genes. The Human Genome Project estimates that around 30,000 human genes provide the codes for the proteins that create each person’s unique anatomical and physiological identity.

The human genome contains approximately three billion base pairs as subunits of deoxyribonucleic acid nucleotide monomers. The sequence of these base pairs forms the code of each gene, and each gene provides the transferable data for one or more proteins.

The Forkhead box protein P2 (FOXP2) gene encodes a transcription factor. The FOXP2 gene is found at the same chromosomal loci in every human cell (except mature red blood cells), but is only expressed in the brain, the gut and in the lung. This particular transcription factor binds with DNA, but is not limited to a single function as it has the ability to bind with hundreds of DNA promotors and therefore, as previously mentioned, can contribute to the production of more than one protein. However, one of FOXP2’s primary functions is in human development of speech and language. We know this, because mutations in the FOXP2 gene lead to ‘autosomal dominant speech and language disorder with orofacial dyspraxia’, or SPCH1.

The BRCA gene mutation is well known as a cause of breast cancer. Usually, the BRCA genes stop tumor formation by repairing DNA damage caused by pollution, diet, lifestyle habits such as smoking, exposure to radiation, and many other factors. In humans with mutated or damaged BRCA genes, this protection no longer applies. Men and women with BRCA mutations either at locus 17q21 (BRCA 1) or 13q12.3 (BRCA 2) have a much higher risk of developing breast cancer, and females have a higher risk of developing ovarian cancer.

The COL1A1 gene encodes a single component of alpha-1 type I collagen, a protein found in many types of connective tissue. This gene can be found at locus 17q21.33. This ‘address’ refers to the COL1A1 gene’s position on the 17 th chromosome, more specifically on the longer ‘q’ arm, and in region 2, band 1, sub-band 33.

MTHFR gives the code the human body requires in order to manufacture methylenetetrahydrofolate reductase. A mutation in the MTHFR gene is actually quite common, and this result means hindrance or an inability to carry out steps within the process of manufacturing end products such as homocysteine and the nucleoside thymidine. This can lead to hyperhomocysteinemia which leads to certain vitamin B deficiencies one of these, vitamin B9 (folic acid), is necessary for embryonal neural development.

CXorf38 is another gene which codes proteins for tissue formation. Cxorf38 is predominantly expressed in glands and lymph nodes and can be found at locus Xp11.4, which indicates the X chromosome (non-autosomal chromosome or sex chromosome), the shorter ‘p’ arm, region 1, band 1, sub-band 4. This locus is pictured below.

What is a Gene?

A gene is a part of a long polynucleotide sequence of the DNA that codes for various proteins that can express a particular genetic trait in the living body.

A gene is basically the physical and functional unit of heredity that stores a particular genetic trait.

Genes can vary in size from having the DNA polynucleotide chain with a few hundred DNA bases to more than 2 million bases.

Genes are part of the chromosomes, which are present in the cell nucleus. A single chromosome can contain hundreds to thousands of genes. Genes are the controller of the inheritance of genetic traits.

Alleles are forms of the same gene with small differences in their sequence of DNA bases and controls the variation in a living body. These small differences contribute to each person’s unique physical features. Just like the gene for height can have the allele of being tall or short if expressed.

In sexually reproducing organisms, everyone has two copies of each gene, one inherited from each parent. During the formation of gametes, one copy of each gene gets passed to one gamete and this leads to the formation of diploid organisms.

In asexually reproducing organisms, the genes are just multiplied and copied to offspring during the reproduction time. This lead to the production of identical offsprings.

In humans, there are about 20,000 and 25,000 genes present in the 23 pairs of chromosomes altogether.

In the case of yeast, there are a total of 6,275 genes on 16 chromosomes that consists of 12 million base pairs of DNA chain.

For example, there are several different alleles for eye color genes, such as blue alleles (blue eyes) and brown alleles (brown eyes).

Genetics, Class 12, Biology | EduRev Notes


Genetics term was given by W. Bateson. W. Bateson is Father of Modern Genetics.

Genetics = Collective study of heredity & Variations.

Heredity = Transmission of genetic characters from parent to offsprings.

Variation = individuals of same species have  some differences, these are called variation.

Muller – Proposed the term "Cytogenetics" (Cytology + Genetics) Father of Actinobiology Actinobiology - study the effect of radiation on living organisms.

Morgan – Father of Experimental genetics He experiment on Drosophila & proposed various concepts.

Gene theory - According to gene theory genes are linearly located on chromosome.

Linkage term, Theory of sex linkage, Crossing over term, Criss - cross inheritance, Linkage map on Drosophila given by Morgan.

  • A. Garrod = Father of human genetics & Biochemical genetics. Garrod discovered first human Metabolic genetic disorder which is calledalkaptonuria(black urine disease). In this disease enzyme homogentisic acid oxidase is deficient. Gave the concept 'One mutant gene one metabolic block'  


To explain the like begets like (offsprings are similar to their parents) several theories were given. They are collectively known as Theories of Blending Inheritance. Some of them are as follows –

1. Vapour fluid theory – Greek philosopher Pythagoras [500B.C.] proposed this theory.

According to this theory, at the time of coitus of male and female, moist vapour secretes from the brain and due to this offsprings are similar as their parents.

2. Semen theory :- This theory has given by Empedocles.

According to his view, the semen of male and female is mixed during coitus. Characters of parents appear into the offsprings due to the mixture.

According toAristotle - a semen of male is considered as "highly purified blood". Which has power of life and it is nourished by semen of female.

3. Preformation theory :- According to this theory germinal [reproductive] cell contains "a miniature figure of a man"

According to Swammerdam  preformation of a miniature of man is found inside the egg is called Mankin.

Those scientists  who believed in the hypothesis of Swammerdam are known as Ovists. On contrary, according to Hartsoeker preformed miniature of man is present in sperm.

A miniature of man is present in sperm, he called - Homunculus Scientists, who believed in  Hart soeker view, are called "spermist."

4. Encasement theory :- This theory was proposed by Charles Bonnet.

According to his view, body of female is just like a chinese box. The all future progenies are packed in the body of female like chinese box.

5. Epigenesis theory :- This theory was proposed by K.Wolf.

According to his view, germinal cells possess an undifferentiate material. This material develops step by step [gradually] after the fertilization. Such type development is known as Epigenesis.

6. Pangenesis theory :- The theory of pangenesis was described by C.Darwin.

This theory postulated that all part of a living body [tissues] synthesize "micro molecules." these micro molecules are known as Pangene or Gemmules.

The male and female pangenes fuse together during the fertilization  these are, further again distributed in the various organs of the body at the time of development.

7. Germplasm theory :- The view was proposed by A.Weisman (1886).

According to him living body of an individual possess two different types of fluid material - Somatoplasm and Germplasm.

Somatoplasm does not participate in the formation of germinal cells.  Therefore, variations are not transferred into the progeny. somatoplasm is mortal because eventually it dies.  

Experiments performed by Mendel on genetics and description of mechanisms of hereditory processes and formulation of principles are known as Mendelism.

Mendel postulated various experimental laws in relation of genetics.

Gregor Johann Mendel (1822 - 1884) :- Mendel was born on July 22,1822 at Heinzendorf  in Austria at Silesia village. Mendel worked in Augustinian Monastery as monk at Brunn city, Austria.

In 1856-57, he started his historical experiments of heredity on pea(Pisum sativum) plant. His experimental work continued  on pea plant till 1865 (19 th century).

The results of his experiments were published in the science journal, "Nature For schender varein" in 1866.

This  journal was in Germen language. Title was "verschue uber Pflangen Hybridan".

This journal was published by 'Natural History society of Bruno'.

A paper of Mendel by the name of "Experiment in plant Hybridization" published in this journal.

Mendel was unable to get any popularity. No one understood of him. He died in 1884 without getting any credit of his work (due to kidney disease (Bright disease) After 16 years of Mendel's death in 1900, Mendel's postulates were rediscovered.

Rediscovery by three scientists independently.

1. Carl Correns - Germany - (Experiment on Maize)

2. Hugo deVries (Holland) (Experiment on Evening Primerose) He republished  the Mendel's results in 1901 in Flora magazine

3. Erich von Tschermak Seysenegg - (Austria) (Experiment on different flowering plants)

The credit of rediscovery of Mendelism goes to three scientists.

Correns gave two laws of Mendelism.

Law of Heredity/Inheritance/Mendelism

I st Law - Law of segregation.

II nd Law - Law of independent assortment.

Mendel experiments remain hidden for 34 years.

Mendel results remain hidden due to :

1. At that time Darwin's book "Origin of Species" published. Scientists  were busy in discussion with this book.

2. Mendel's ideas were ahead of that time.

3. Mendel used higher statistical calculation in his experiments so the results were complicated to understand.

4. Mendel also performed his experiments on Hieraceum plant on suggestion of Karl Nageli but Mendel did not get succeed because in Heiracium, Parthenogenesis is present.

Reasons for Mendel's success :

1. Mendel studied the inheretance of one or two characters at a time unlike his predecessors who had considered many characters at a time. (Kolreuter-Tobacco plant, John Goss & Knight -Pea plant).

Selection of garden Pea plant is suitable for studies which have the following advantages :
(i) Pea plant is annual plant with short life cycle of 2-3 months so large no. of offsprings can be analysed within a
short period of time.
(ii) It has many contrasting traits.
(iii) Natural self pollination is present in pea plant.
(iv) Cross pollination can be performed in it artificially so hybridization can be made possible.
(v) Pea plant easy to cultivate.
(vi) Pea seeds are large. In addition to pea, Mendel worked on rajama and honey bee.
3. Mendel quantitatively analyse the inheritance of qualitative characters.
4. He maintained the statistical records of all the experiments.
Mendel's work : Mendel studied 7 characters or 7 pairs of contrasting traits.

Average of all traits studied 2.98: (= 3:1)

Gene which controls more than one character is called as pleiotropic gene.

In Wrinkled seed free sugar is more in place of starch.

Special Point :
S. Blixt concluded that the genes studied by Mendel are located on four different pairs of chromosomes.

Pod colour --------------  Ch. no. 5 th
Seed form ----------------- Ch. no. 7 th
Two of the genes are on chromosome 1 st and three are on chromosome 4, genes are located far apart on the chromosome except genes controlling plant height and pod shape.
Mendel did not study the gene controlling plant height and pod shape so Mendel did not detect linkage.

Technique of Mendel

He developed a technique Emasculation and Bagging for hybridization in plants.

Flowers of pea plant are bisexual. In this method one considered as male and another as female.

Stamens of the plant which is used as female, are removed at juvenile stage, this is called Emasculation.

Emasculation is done to prevent self pollination.

Emasculated flowers covered by bags, this is called bagging.

Bagging is only used to prevent undesirable cross pollination.

Mature pollen grains are collected from male plants and spread over emasculated flower.

Seeds are formed in the female flower after pollination.

The plants that are obtained from these seeds are called First Filial generation or F1 generation according to Mendel.

Mendel was great plant breader(true breader).

1. Factors :- Unit of heredity which is responsible for inheritance and appearance of characters.

These factors were referred as genes by Johannsen(1909). Mendel used term "element" for factor.

Morgan first used symbol to represent the factor. Dominant factors are represented by capital letter while recessive factor by small letter.

2 Allele :- Alternative forms of a gene which are located on same position [loci] on the homologous chromosome is called Allele. Term allele was coined by Bateson.

3. Homozygous :- A zygote is formed by fusion of two gametes having identicle factors is called homozygote and organism developed from this zygote is called homozygous.

4. Heterozygous :- A zygote is formed by fusion of two different types of gamete carrying different factors is called heterozygote (Tt, Rr) and individual developed from such zygote is called heterozygous.

The term homozygous and heterozygous are coined by Bateson.

5. Hemizygous :- If individual contains only one gene of a pair then individual said to be Hemizygous. Male individual is always Hemizygous for sex linked gene.

6. Phenotype :- It is the external and morphological appearance of an organism for a particular character.

7. Genotype :- The genetic constitution or genetic make-up of an organism for a particular character.

Genotype & phenotype terms were coined by Johannsen.

8. Phenocopy :- If different genotypes are placed in different environmental conditions then they produce same
phenotype. Then these genotypes are said to be Phenocopy of each other.

When we consider the inheritance of one character at a time in a cross this is called monohybrid cross. First of all, Mendel selected tall and dwarf plants.

Checker Board Method :

First time, it was used by Reginald. C. Punnett (1875 - 1967)
The representation of generations to analyse in the form of symbols of squares. Male gamets lie horizontally and female gametes lie vertically.

T T = Tall (dominant homozygous),

T t = Tall (dominant heterozygous),

t t = Dwarf (recessive homozygous).

The ratio of characters (traits) appear/ visible morphologically is phenotypicratio. It is 3: 1. Genetic constitution is called Genotype [using symbols for genes] it is 1 : 2 :1

Conclusions (results) of Monohybrid Cross

I st Conclusion (Postulate of paired factors) :

According to Mendel each genetic character is controlled by a pair of unit factor. It is known as conclusion of
paired factoror unit factor.

II nd Conclusion (Postulate of Dominance):
This conclusion is based on F1 - generation. When two different unit factors are present in single individual, only one unit factor is able to express itself and known as dominant unit factor. Another unit factor fails to express is the recessive factor. In the presence of dominant unit factor recessive unit factor can not express and it is known as conclusion of dominance.

III rd Conclusion (Law of segregation):

During gamete formation the unit factors of a pair segregate randomly and transfer inside different gamete.

Each gamete receives only one factor of a pair so gametes are pure for a particular trait. It is known  as conclusion of purity of gametes or segregation.

  • There is no exception of Law of segregation. The segregation is essential during the meiotic division in all sexually reproducing organisms. (Nondisjunction may be exception of this law).


A cross in which study of inheritance of two pairs of contrasting traits.

Mendel wanted to observe the effect of one pair of heterozygous on other pair.

Mendel selected traits for dihybrid cross for his experiment as follows :-

[1] Colour of cotyledons→ Yellow (Y)  & Green (y)

[2] Seed form → Round (R) and Wrinkled (r) yellow and round characters are dominant and green and wrinkled are recessive characters.

Mendel crossed, yellow and round seeded plants with green and wrinkled seeded plants.

All the plants in F1–generation had yellow and round seeds.

When F1 plants were self pollinated to produce four kinds of plants in F2 generation such as yellow round, yellow–wrinkled, green round and green wrinkled,  there were in the ratio of 9 : 3 : 3 : 1. This ratio is known as dihybrid ratio.

Expression of yellow round (9) and green wrinkled (1)  traits shows as their parental combination.

Green Round and yellow wrinkled type of plants are produced by the results of new combination.

Demonstration by checker board method :-

F2 - Generation 

Thus, Phenotypic Ratio = 9 : 3 : 3 : 1

Homozygous yellow & Homozygous Round – YY RR = 1

Homozygous yellow & Heterozygous Round – YY Rr = 2

Heterozygous yellow & Homozygous Round – Yy RR = 2

Heterozygous yellow & Heterozygous Round – Yy Rr = 4

Homozygous yellow & Homozygous wrinkled – YY rr = 1

Heterozygous yellow & Homozygous wrinkled – Yy rr = 2

Homozygous green & Homozygous Round – yy RR = 1

Homozygous green & Heterozygous Round – yy Rr = 2

Homozygous green & Homozygous wrinkled – yy rr = 1

Thus, Genotypic Ratio = 1:2:2:4:1:2:1:2:1

Fork line method -To find out the composition of factors inside the gamete, we use fork line method.

Type of gamete / phenotypic category = 2 n

 n = No of hybrid character or heterozygous pair.

eg in dihybrid cross = 32 = 9 genotype

No. of zygote produced by selfing of a gen otype = 4 n

Conclusion (Law of Independent Assortment): The F2 generation plant produce two new phenotypes, so inheritance of seed colour is independent from the inheritance of shape of seed. Otherwise it can not possible to obtain yellow wrinkled and green round type of seeds.

This observation leads to the Mendel's conclusion that different type of characters present in plants assorted independently during inheritance.

This is known as Conclusion of Independent Assortment. It is based on F2 - generation of dihybrid cross.

The nonhomologous chromosome show random distribution during anaphase-I of meiosis.

Explaination :-

A pure yellow and round seeded plant crossed with green and wrinkled seeded plant which are having genotype YYRR and yyrr to produced F1 generation having YyRr genotype.

Both the characters recombine independently from  each other during gamete formation in F1 generation .

Factor (R) of pair factor (Rr) is having equal chance to (Y) factor or (y) factor of gametes during recombination to form two type of gametes (YR) and (yr).

Similarly (r) factor also having equal chance with (Y) factor or (y) factor of gametes to form a two type gametes - (Yr) and (yr).

Thus, total four types of gametes - (YR), (yR), (Yr), and (yr) are formed.

Therefore, during the gametes formation in F1 generation , independent recombination is possible.

– The law of independent assortment is most criticised. Linkage is the exception of this.

A back cross is a cross in which F1 individuals are crossed with any of their parents.

(1) Out Cross : When F1 individual is crossed with dominant parent then it is termed out cross. The generations obtained from this cross, all possess dominant character. so the any analysis can not possible in F1 generation.

[2] Test Cross : When F1 progeny is crossed with recessive parent then it is called test cross. The total generations obtained from this cross, 50% having dominant character and 50% having recessive character. [Monohybrid test cross]. Test cross helps to find out the  genotype of dominant individual.

[a] Monohybrid Test Cross :- The progeny obtained from the monohybrid test cross are in equal proportion , means 50%  is dominant phenotypes and 50% is recessive phenotypes.

It can be represented in symbolic forms as follows.

[b] Dihybrid Test Cross:- The progeny is obtained from dihybrid test cross are four types and each of them is 25%.

   The ratio of Dihybrid test cross = 1:1:1:1

Conclusion:-  In test cross phenotypes and genotypes ratio are same.  

When two parents are used in two experiments in such a way that in one experiment "A" is used as the female parent and "B" is used as the male parent, in the other experiment "A" will be used as the male parent and "B" as the female parent. such type of a set of two experiments is called Reciprocal cross.
Characters which are controlled by karyogene are not affected by Reciprocal cross. In case of cytoplasmic inheritance result change by Reciprocal cross.



Gene interaction is two types :

(i) Allelic interaction/Intragenic interaction

(ii) Non allelic interaction/Intergenic interaction

(i) Allelic interaction/Intragenic interaction: Allelic interaction takes place between allele of same gene which are present at same locus.

Example of Allelic interaction are as follows :–

[1] Incomplete dominance :- According to Mendel's law of dominance, dominant character must be present in F1 generation. But in some organisms, F1 generation is different from the both parents.

Both factors such as dominant and recessive are present in incomplete dominance but dominant factors is unable to express its character completely, resulting Intermediate type of generation is formed  which is different from the both parents. Some examples are –

  • (a) Flower colour in Mirabilis jalapa : Incomplete dominance was first discovered by Correns in Mirabilis jalapa. This plant is called as Ɗ O' clock plant 'or'Gul-e-Bans'. Three different types of plant are found in Mirabilis on the basis of flower colour, such as red , white and pink.

& When plants with red flowers is crossed with white flower, plants with pink flower obtained in F1 generation. The reason of this is that the genes of red colour is incompletely  dominant over the genes of white colour.

& When, F1 generation of pink flower is self pollinated then the phenotypic  ratio of F2 generation  is red, pink, white is 1:2:1 ratio in place of  normal monohybrid cross ratio 3:1.

& The ratio of phenotype and genotype of F2 generation in incomplete dominance is always same.

(b) Flower colour in Antirrhinum majus :- Incomplete dominance is also seen in flower colour of this plant.This plant is also known as 'Snapdragon ' or 'Dog flower'. Incomplete dominance is found in this plant which is the same as Mirabilis.

(c) Feather colour in Andalusian Fowls :- Incomplete dominance is present for their feather colour.

When a black colour fowl is crossed with a white colour fowl, the colour of F1 generation  is blue.

[2] Co-dominance :- In this phenomenon, both the gene expressed for a particular character in F1 hybrid progeny. There is no blending of characters, wherease both the characters expressed equally.

Examples :- Co-dominance is seen in animals for coat colour. when a black parent is crossed with white parent, a roan colour F1 progeny is produced.

When we obtain F2 generation from the F1 generation, the ratio of black black-white (Roan) white animals is  1 : 2 : 1

Note :-  F2 generation is obtained in animals by sib-mating cross.


It is obvious by above analysis that the ratio of phenotype as well as genotype is 1:2:1 in co-dominance.

Sp. Note :- In incomplete dominance, characters are blended phenotypically, while in co-dominace, both the genes of a pair exhibit both the characters side by side and effect of both the character is independent from each other.

Other Examples of Co-dominance :
(ii) AB blood group inheritance (I A I B )
(iii) Carrier of Sickle cell anaemia (Hb A Hb S )

[3] Multiple allele :– More than 2 alternative forms of same gene called as multiple allele. Multiple allele is formed due to mutation. Multile allele located on same locus of homologous chromosome.

A diploid individual contains two alleles and gamete contains one allele for a character.

Ex. Blood group - 3 alleles Coat colour in rabbit - 4 alleles

If n is the number of allele of a gene then number of different possible genotype = 

Example  of multiple allele :

1. ABO blood group → ABO blood groups are determined by allele I A , allele I B , allele I O

I O = recessive Possible phenotypes - A, B, AB, O

2. Coat colour in rabbit → Four alleles for coat colour in rabbit

Wild type = Full coloured = agouti = C +

Himalayan [white with black tip on extremities (like nose, tail and feet)] = c h

 Chinchilla [mixed coloured and white hairs] = c ch

These alleles show a gradient in dominance  C + > c ch > c h > c a

Coloured = C+C+, C+c ch , C+c h C+c a

Chinchilla = c ch c ch , c ch c h , c ch c a

Himalayan = c h c h , c h c a

Possible genotype  =    = 10 genotypes

Eye colour in Drosophila  and self incompatibility genes in plants are also the example of multiple allelism.

[4] Lethal gene :– Gene which causes death of individual in early stage when it comes in homozygous condition called lethal gene. Lethal gene may be dominant or recessive both, but mostly recessive for lethality. Many of these genes which do not cause definite lethality are called semilethals. In semilethal gene death occurs in late stage.

1. Lethal gene was discovered by L. Cuenot in coat colour of mice.
Yellow body colour(Y) was dominant over normal brown colour(y).
Gene of yellow body colour is lethal.
So homozygous yellow mice are never obtained in population. It dies in embryonal stage.
When yellow mice were crossed among themselves segregation for yellow and brown body colour was obtained in 2 : 1 ratio.

YY - death in embryonal stage modified ratio = 2 : 1

2. In plant lethal gene was first discovered by E. Baur in Snapdragon (Antirrhinum majus)

Homozygous golden leaves are never obtained.

3. Sickle cell anaemia in human. In human, gene of sickle cell anaemia HbS is the example of lethal gene.
When two carrier indivudials of sickle cell anaemia are crosed then offsprings are obtained in 2 : 1 ratio.

Sublethal gene but ratio 2 : 1

[5] Pleiotropic gene :– Gene which controls more than one character is called pleiotropic gene. This gene shows multiple phenotypic effect.
For example :

(1) In Pea plant : Single gene influences 

2) In Drosophila recessive gene of vestigial wings also influence the some another characters–

  • Structure of reproductive organs
  • Longevity (Length of Body)
  •  Bristles on wings.
  • Reduction in egg production.

(3) Examples of  pleiotropic gene in human.

(a) Sickle cell anaemia - Gene Hb S β provide a classical example of pleiotrophy. It not only causes haemolytic anaemia but also results increased resistance to one type of malaria that caused by the parasite Plasmodium falciparum. The sickle cell Hb S β allele also has pleiotropic effect on the development of many tissues and organs such as bone, lungs, kidney, spleen, heart.

(b) Cystic fibrosis – Hereditary metabolic disorder that is controlled by a single aoutosomal recessive gene.
The gene specifies an enzyme that produces a unique glycoprotein.
This glycoprotein results in the production of mucous.
More mucous interfere with the normal functioning of several exocrine glands including those in the skin, lungs, liver and pancreas.

(ii) Non allelic interaction/Intergenic interaction When interaction takes place between non allele is called non allelic gene interaction. It changes or modifies other non allelic gene.
Examples of nonallelic interaction.

1. Epistasis :- When, a gene prevents the expression of another non-allelic gene, then it is known as epistatic gene and this phenomenon is known as Epistasis.
Gene which inhibit the expression of another non alleleic gene is called epistatic gene and expression of gene which is suppressed  by epistatic gene called hypostatic gene.

Hair Colour in Dog :-

B = Dominant allele for black colour of hairs.
b = Recessive allele for brown colour of hairs.
I = Epistatic gene.
If the genotype bbii for brown colour and BBII for white colour.
Following types of generation will be obtained by following crosses.

It is obviously clear by above analysis, the phenotypic ratio of F2 - generation in epistasis is  - 12:3:1

2. Inhibitory gene - Inhibitory gene itself have no phenotype but inhibits the effect of other non allelic gene. Non allelic gene behaves as  recessive. * Inhibitory gene must be in dominant stage & inhibit the effect of only dominant gene.
Ex., Leaf colour in Rice
R – Purple
r – Green
I – Inhibitory gene
R – I – Green – 9
R – ii – Purple – 3
rr – I – Green – 3
rr – ii – Green – 1
13 (Green) : 3(Purple)

3. Complementary Gene :- Two pair of non allelic genes are essential in doninant form to produce a particular character.
Such genes that act together to produce an effect that neither can produce, it's  effect separately are called complementary genes.
Both types of gene must be present in dominant form.

Example :- Colour of flowers in Lathyrus odoratus :-

Thus phenotypic ratio of complementary genes = Coloured : Colourless   9  : ه

4. Duplicate Genes :-

Two pairs of non-allelic genes require  are for a character . If any one of them gene is dominant, then this character is expressed such type of gene is called duplicate gene.

Example :- Fruit shape in Capsella. Two pair of non-allelic genes are present in Capsella for triangular shape of fruits.

If any one gene out of them is dominant, the shape of fruit is triangular and no one gene is dominant than fruits will be elongated.

ttdd = For top shape of fruits

Phenotypic ratio of F2 -> Triangular : Top shaped 15    :   1

5. Additive Gene effect : In additive gene effect if non allelic gene seperately in dominant stage phenotype is same but both gene come dominant stage together phenotype is change due to additive effect. eg. Fruit shape in cucumber

A – B – discoid (new phenotype)

6. Collaboratory Gene :- Two pairs of non-allelic gene interacting together to produce a new phenotypic character.

Example :- Comb - shape in Chickens -

Both R & P = For walnut comb

A new type of phenotype walnut - (Rr Pp) comb is produced by the cross in between Rose comb (RR pp) and Pea comb (rr PP)


Thus, phenotypic ratio of collaboratory gene = Walnut : Rose : Pea : Single   = 9 : 3 : 3 : ف

7. Supplementary gene or Recessive Epistasis :- A pair of gene change the effect of another non allelicgene, is called supplementary gene.
Example :- Coat colour in Mice.
If alleles,   C = Black coat colour                  

c = Albino (Colourless coat) or (It has no effect)                  
A = Supplementary geneWhen black coat mice crossed with albino mice, the F1 generation is Agouti.
It means, here the effect of non allelic gene is changed.

Thus, Recessive epistasis or supplementary gene ratio in F2 -  Agouti : Black : Albino

Inheritance of characters in which one character is controlled by many genes and intensity of character depends upon the number of dominant allele.
Polygenic inheritance first described byNilsson - Ehlein kernal colour of wheat.
Nilsson - Ehle said that kernal colour of wheat is regulated by two pairs of gene.

Example-2. :- Colour of the skin in Human.
The inheritance of colour of skin in human studied by Devenport.
Five types of phenotype of colour of skin are found in human.
When a Negro (AA BB) phenotype crossed with white (aa bb) phenotype, intermediate phenotype produced in F1 generation . Phenotypes of F2 generation as follows.

Phenotypic ratioof F2 generation of quantitative inheritance as

  • In new discovery human skin colour and kernal colour in wheat is regulated by 3 pairs of alleles so phenotypic ration of F2 generation.

Inheritance of characters which are controlled by cytogene or cytoplasm is called cytoplosmic inheritance. Genes which are present in cytoplasm called 'cytogene' or 'plasmagene' or extra nuclear gene.
Total cytogene present in cytoplasm is called 'Plasmon'.
A gene which is located in the nucleus is called 'karyogene'.

  • Inheritance of cytogene in organisms occurs only through the female. Because female gamete has karyoplasm, simultaneously it has cytogene because of more cytoplasm.
  • The male gamete of higher plant is called male nucleus. It has very minute [equivelent to nil] cytoplasm. so male gamete only inherited karyogene.
  • Thus, inheritance of cytogene occurs only through female. (also called maternal inheritance)
  • If there is a reciprocal cross in this condition, then results may be effected.

Cytoplasmic inheretance are of three types :

1. Cytoplasmic inheritance involving essential organelles like, Chloroplast and mitochondria called as organellar genetics.

2. Maternal effect depending indirectly on nuclear genes and involving no known cytoplasmic hereditary unit called aspredetermination. In this maternal effect is determined before fertilization.

3. Cytoplasmic inheritance involving dispensable and infective hereditary particle in cytoplasm which may or may not depend on nuclear genes called as Dauermodification.

Example of Organellar Genetics : (True examles of cytoplasmic inheritance)
(a) Plastid inheritance in Mirabilis jalapa – cytoplasmic inheritance first discovered by Correns in Mirabilis jalapa. InMirabilis jalapa branch (leaf) colour is decided by type of plastid present in leaf cells. So it is an example of cytoplasmic inheritance.
Branch colour

(b) Male sterility in maize plant : Gene of male sterelity present in mitochondria. If a normal male plant crossed with a female plant which has genes of male sterility then all the generation of male become sterile because a particular gene was present with female which inherited by female.

(c) Albinism in plant : Gene of albinism found in chloroplast. Gene of albinism in Maize is lethal.

(d) Inheritance of Bacterial plasmid : In bacteria plasmid inheritance is due to conjugation.

(e) Petite form in yeast (mitochondrial gene) : Petite is mutant form of yeast. This mutant form  is slow growing on culture medium.

(f) Iojap inheritance in Maize : Iojap is characterized by constrasting strip of green and white colour of leaves.

(g) Poky Neurospora (mitochondrial gene) : Poky is mutant form of Neurospora. It is slow growing on culture medium.
Example of predetermination

Shell coiling in snail (Limnaea peregra) In snail shell coiling can be of dextral (Coiling to the right) or sinistral (coiling to the Left). This direction of coiling is genetically controlled. The dextral coiling  depends upon dominant allele 'D' and sinistral coiling depends upon recessive allele 'd'. So the dextral is DD, Dd and sinistral is dd.


Above reciprocal cross indicates that phenotype of offspring is decided by genotype of female parent not the phenotype of female parent. Even if female parent contains only one dominant gene 'D' then phenotype of all offsprings is dextral.
Example : 

Example of Dauermodification -

(a) Sigma particle in Drosophila:- These particles are virus like particles which are present in Drosophila and related to CO2 sensitivity. Inheritance of sigma particle takes place through the egg cytoplasm.

(b) Kappa particle in Paramecium:- Kappa particles are found in certain "Killer strains" of Paramecium and are responsible for production of substance paramecin which is toxic to strain not prossessing Kappa. (Sensitive Strain) The minimum number of kappa particles is  required 400 to secrete paramecin. Kappa particles are symbiotic bacteria named "Caedobacter taeniospiralis".


This theory was proposed by Walter Sutton and Theodor Boveri (1902). Following are the main points of this theory

1. Gametes serve as the bridge between two successive generations.

2. Male and Female gametes play an equal role in contributing hereditary components of future generation.

3. Only the nucleus of sperm combines with ovum. Thus, the hereditary information is contained in the nucleus.

4. Chromatin in the nucleus is associated with the cell division in the form of chromosomes.

5. Any type of deletion or addition in the chromosomes can cause structural and functional changes in living beings.

6. A sort of parallelism is observed between Mendelian factors and chromosomes.

7. A number of genes or Mendelian factors are found in each chromosome.

8. Determination of sex in most of the animals and plants is affected by specific chromosomes. These chromosomes are called sex chromosomes.

Parallelism Between Gene and Chromosomes

1. Chromosomes are also transferred from one generation to the next as in the case of genes (Mendelian factors).

2. The number of chromosomes is fixed in each living species.These are found as homologous pairs in diploid cells.

One chromosome from father and the other contributed by the mother constitute a homologous pair.

3. Before cell division, each chromosome as a whole and the alleles of genes get replicated and are separated during mitotic division.

4. Meiosis takes place during gamete formation. Homologous chromosomes form synapses during prophase-I stage which in later course get separated and transferred to daughter cells. Each gamete or a haploid cell has only one allele of each gene present in the chromosome.

5. A characteristic diploid number is again established by the union of the two haploid gametes.

6. Both chromosomes and the alleles (Mendelian factors) behave in accordance to Mendel's law of segregation.

In the homologus chromosomes of a pure tall plant, allele (T) is found for tallness in each chromosome. Likewise, in a pure dwarf plant (tt), allele (t) is present in each chromosome.

These homologous chromosomes get separated during meiotic divisoin. Hence, each gamete possesses only one chromosome of an each pair. Accordingly, all the gametes of tall plants possess a chromosome with an allele of tallness (T), while the gametes of dwarf plants possess a chromosome with an allele for dwarfness (t). Their cross to produce F1 generation will yield tall hybrid plants with homologous chromosomal pair containing Tt allelic pair. In this generation two kinds of gamete will be formed during gametogenesis, 50% with the allele (T) for tallness and 50% with the allele for dwarfness (t).Random combination of these gametes will produce offsprings in F2 generation in the ratio of 25% pure tall (TT), 50% hybrid tall (Tt) and 25% dwarf (tt)

Collective inheritance of character is called linkage. Linkage first time seen byBatesonandPunnett inLathyrus odoratus and gave coupling and repulsion phenomenon. But they did not explain the phenomenon of linkage. Sex linkage was  first discoverd by Morgan in Drosophila & coined the term linkage. He proposed the theory of linkage.

Linkage and independent assortment can be represented in dihybrid plant, as –

In case of linkage in dihybrid AaBb

In case of independent assortment in dihybrid AaBb

Theory of linkage

1. Linked genes are linearly located on same chromosome. They get separated if exchange (crossing over), takes place between them.

2. Strength of linkage ⓫/ distance between the genes . It means, if the distance between two genes is increased then strength of linkage is reduced and it proves that greater is the distance between genes, the greater is the probability of their crossing over.

Crossing over obviously disturbs or degenerates linkage. Linked genes can be separated by crossing over.

Factors affecting crossing over (C.O) :-

(1) Distance ­ />= C.O.­  />

(2) Temperature ­ />= C.O.­  />

(3) X-Ray  />­ =  C.O.­  />

(4) Age ­ =  C.O.

(5) Sex - Male C.O.  (Crossing over totally absent in male Drosophila.)

Arrangement of linked Genes on Chromosomes :-The arrangement of linked genes in any dihybrid plant is two types.

[a] Cis - Arrangement :- When, two dominant genes located on one chromosome and both recessive genes located on another chromosome, such type of arrangement is termed as cis-arrangement. Cis-arrangement is  an original arrangement.

[b] Trans-arrangement :- When a chromosome bears one dominant and  one recessive gene, and another chromosome also possess one dominant and one recessive gene, such type of arrangement is called trans-arrangement.Trans-arrangement is not an original form. It is due to crossing over. Two types of gamete also formed in trans-arrangement but it is different from cis-arrangement (Ab) and (aB).

Types  of Linkage :- There are two types of linkage –

1 COMPLETE LINKAGE :- Linkage in which genes always show parental combination. It never forms new combination.

Crossing over is absent in it. Such genes are located very close on the chromosomes. Such type of  linkage very rare in nature. e.g. male Drosophila, female silk moth.

2. INCOMPLETE LINKAGE :- When new combinations also appear along with parental combination in offsprings, this type of linkage is called incomplete linkage, the new combinations form due to crossing over. The percentage of new combination is equal to the percentage of crossing over.(អ%)

Linkage group :- All the genes which are located on one pair of homologous chromosome form one linkage group. Genes which are located on homologous chromosomes inherit together so we consider one linkage group.

Application of Linkage :-

Distance can be identified by the incomplete linkage. It's unit is centi Morgan.  

Genetic map/Linkage map/chromosome map - In genetic map different genes are linearly arranged according to % of crossing over (μ Distance) between them. With the help of genetic map we can find out the position of a particular gene on chromosome. Genetic map is helpful in the study of genome.


When the genes are present on sex-chromosome is termed as sex linked gene and such phenomenon is known as sex-linkage. Two - types of sex linkage :

1. X-linkage.
Genes of somatic characters are found on x-chromosome. The inheritance of x-linked character may be through the males and females. e.g. Haemophilia, Colour blindness

2. Y- linkage - The genes of somatic characters are located on Y- chromosome. The inheritance of such type of character is only through the males. Such type of character is called Holandric character. These characters
found only in male.

e.g. (1) Gene which forms TDF /sry-gene                   (3) Webbed toes
(2) Hypertrichosis (excessive hair on ear pinna.)       (4) Porcupine skin
Gene which is located on differential region of Y - chromosome is known as Holandric gene.

Example of X-Sex linkage :-
[i] Eye colour in Drosophila
:- Eye colour in Drosophila is controlled by a X–linked gene.

If a red eyed colour gene is represented as '+' and white eyed colour represented as 'w', then on basis of this
different type of genotypes are found in Drosophila.

Gene for red eye is dominant (+) and white colour of eye is recessive (w)

Homozygous red eyed female = X + X +
Heterozygous red eyed female = X + X w
Homozygous white eyed female = X w X w
Hemizygous red eyed male = X + Y
Hemizygous white eyed male = X w Y

It is clear by above different types of genotype that female either homozygous or heterozygous for eye colour.
But, for the male eye colour, it is always hemizygous.

[ii] Haemophilia :-

Haemophilia is also called "bleeder's disease" and first discovered by John Otto (1803). The gene of
haemophilia is recessive and x-linked lethal gene.

On the basis of x-linked, following types of genotype are found.

X h X = Carrier female
X h X h = Affected female
X h Y = Affected male.

But, X h X h type of female dies during embryo stage because in homozygous condition, this gene becomes lethal and causes death.

Haemophilia -A → due to lack of factor -VIII (Antihaemophilic globulin AHG)

Haemophilia B or Christmas disease - due to lack of factor - IX (Plasma thromboplastin component)

Haemophilia - C (Antosomal disorder) → due to lack of factor - XI (Plasma Thromboplastin antecedent)

[iii] Colour Blindness :- The inheritance of colour-blindness is alike as haemophilia, but it is not a lethal
disease so it is found in male and female.(discovered by Horner)

Three types of colour blindness are-
[a] Protanopia
:- It is for red colour.
[b] Deuteranopia :- It is for green colour
[c] Tritanopia :- For blue colour blindness. Colour blindness is cheked by ishihara - chart.

Other examples of X - sex linkage
[iv] Diabetesinsipidus (recessive).
[v] Duchenne muscular dystrophy (recessive).
[vi] Fragile xsyndrome(recessive).
[vii] Pesudoricketes (Dominant)
[viii] Defective enamel of teeth (Dominant)

Examples of X-Y linkage
(i) Xeroderma pigmentosum
(ii) Epidermolysis bullosa

Types of Inheritance of sex linked characters :-
1. Criss cross inheritance (Morgan)
:- In criss-cross inheritance male or female parent transfer a X- linked character to grandson or grand daughter through the offspring of opposite sex.

(a) Diagenic (Diagynic) :- Inheritance in which characters are inherited from father to the daughter and from daughter to grandson.

Father → daughter → grand son.

(b) Diandric :- Inheritance in which characters are inherited from mother to the son and from son to grand

Mother → Son → Grand-daughter.

(2) Non criss-cross inheritance : In this inheritance male or female parent transfer sex linked character to grand son or grand daughter through the offspring of same sex.
(a) Hologenic (Hologynic) :- Mother → Daughter → Grand-daughter (female to female)
(b) Holandric :- Father → Son → Grand-son (male to male)

Sex-Limited Character :- These characters are present in one sex and absent in another sex. But their genes are present in both the sexes and their expression is depend on sex hormone.

Example :- Secondary sexual characters → these genes located on the autosomes and these genes are present in both male and female, but effect of these are depend upon presence or absence of sex-hormones.

For example - genes of beard-moustache express their effects only in the presence of male hormone - testosterone.

Sex Influenced Characters : - Genes of these characters are also present on autosomes but they are influenced differently in male and female. In heterozygous condition their effect is different in both the sexes.

Example :- Baldness :- Gene of baldness is dominant (B).

Gene Bb shows partiality in male and female, Baldness is found in male due to effect of this gene, but baldness is absent in female with this genotype.


Establishment of sex through differential development in an individual at an early stage of life, is called sex
determination. There are different methods for sex determination in organisms like environmental, non-allosomic genetic determination, allosomic sex determination and haplodiploidy.

Sex Determination on the basis of fertilization.
Three types –
1. Progamic
– Sex is determined before fertilization.
eg. - drone in honey bee

2. Syngamic - Sex is determined during fertilization.
eg. - most of plants & animals

3. Epigamic - Sex is determined after fertilization.
eg. - Female in honey bee.

Environmental Determination of Sex. It is non-genetic determination of sex which is based purely on environmental conditions. The organisms are potentially hermaphrodite and capable of expressing any of the
two sexes.

1. In marine worm Bonellia, larva develops into female if it settles down alone in an isolated place. Any larva
coming in contact with the already grown female, it changes into male, and lives as a parasite in the uterus of

2. Crepidula (marine mollusca) where larva develops into male in the company of female and develops into

3. In crocodiles low temperature induces femaleness and high temperature maleness.

4. ln turtles temperature below 28°C induces maleness, above 33°C femaleness while between 28 - 33°C
equal number of male and female animals are formed.

5. In marine fish Medusa sex changes according to environmental condition, becoming male in cold water and
female in warm water.

Allosomic determination of sex –
Chromosomes are of two types -

(a) Autosomes or somatic chromosomes -
These regulate somatic characters.

(b) Allosomes or Heterosomes or Sex chromosomes -

These chromosomes are associated with sex determination. Term "Allosome" & "Heterosome" were given
by Montgomery.

Sex chromosomes first discovered by "Mc Clung" in grass hopper
X- Chromosome discovered by "Henking" and called 'x-body'.

Wilson & Stevens proposed chromosomal theory for sex determination.

(1) XX - XY type or Lygaeus type :- This type of sex determination first observed by Wilson & Stevens in
Lygaeus insect. Two types–

(a) XX female and XY male :- In this type of sex determination female is Homogametic i.e produces only
one type of gamete

In male X-chromosome containing gametes is called "Gynosperm" and Y- chromosome containing gamete is called "Androsperm".
eg. Man and dioecious plants like Coccinea, Melandrium

(b) XY female and XX male or ZW female and ZZ male :- In this type of sex determination female is
Heterogametic i.e produces two types of gamete and male individual is homogametic i.e produces one
type of gamete.
It is found in some insects like butter flies, moths and vertebrates like birds, fishes and reptiles.
In plant kingdom this type of sex determination is found in Fragaria elatior.

(2) XX female and XO male :- or "Protenor type" :- In this type of sex deternination deficiency of one chromosome in male. In this type, female is homogametic and male is heterogametic.

Example :–
– Grass hopper
– Squash bug Anasa
– Cockroach
– Ascaris and in plants like - Dioscorea sinuta & Vallisneria spiralis

Genic balance theory :- C.B. Bridges proposed genic balance theory for sex determination in Drosophila.
– According to Bridges in Drosophila Y-chromosome is heterochromatic so it is not active in sex determination
In Drosophila sex determination takes place by sex index ratio.

In Drosophila gene of femaleness (Sxl- gene) (Sxl=Sex lethal gene) is located on x-chromosome and gene of
maleness is located on autosome
Gene of male fertility is located on y-chromosome and in Drosophila, y-chromosome plays additional role in
spermatogenesis and development of male reproductive organ, so y-chromosome is essential for the production offertilemale.

(c) X/A = 1.5 → Super female or meta female (sterile) (2A + XXX)

(d) X/A = less than 0.5 → Super male or meta male (Sterile) (3A + XY)

(e) X/A = = In between 0.5 and 1 → Intersex (Sterile) (3A+XX)

Gynandromorph –
Body of some Drosophila has some cells with male genotype (X0) and some cells with female genotype (XX).
Body of such type of Drosophila has half lateral part of male and half lateral part of female and it is called bilateral gynandromorph. It is formed due to loss of one x-chromosome at metaphase plate during first zygotic division. Formation of gynandromorph is the best evidence that y-chromosome does not play any role in sex differentiation. 

 Haploid - diploid mechanism –
In insects of order Hymenoptera which includes ants,honey bees, wasps etc.
Sex determination takes place by sets of chromosomes.
Diploid (two sets) → Female
Haploid (One set) → Male
In honey bee, male individual (Drone) develops from unfertilized eggs (Haploid). Male is always parthenote.
Queen and worker bees develop from diploid eggs i.e. fertilized egg.

Sex determination by Hormone –
Dizygotic twins are common in cattle like cow, sheep, goat etc. Some times the placentae of the two dizygotic
twins fuse forming blood vascular connections between two developing foetus. If twins are dizygotic, one
foetus may be male and the other female.

  •  Male hormone produced before female hormone by male twins which suppresses the differentiation of female internal sex organ. Such a sterile female with Under developed ovaries, oviducts, Uterous etc. is called free martin.  In free martin conditions, female is sterile & male is normal.

Cytological basis of sex determination –
Barr body technique or Lyon's hypothesis -

Interphasic nucleus of human female contains two X- chromosomes. Out of two, one X- chromosome becomes
heterochromatin and other X- chromosome is euchromatin. By staining X- heterochromatin, it appears as a
dense body which is called Barr body. (Facultative hetrochromatin)
No. of Barr body ⇒ (No. of X chromosomes – 1)
So in a Normal female (2A + XX) → One Barr body
Normal male (2A + XY) → Barr body absent
Turner syndrome (Sterile female) (2A + XO) → No. Barr body
Klinefelter syndrom (Sterile male)(2A + XXY) → One Barr body

Drum stick which occurs in blood of female of mammals, is also a type of barr body. Drum stick is absent in
neutrophils of Male.

Sex determination in human –
There occur a special gene on differential region of Y-chromosome of human, called Sry - gene (Sex determine
region on y chromosome ). This gene forms a proteinaceous factor called TDF (testes determining factor). TDF
responsible for the development of male reproductive organs. So presence and absence of Y- chromosome
determines sex.

Sex determination in plant –
H.E. Warmke
discovered sex determination in Melandrium plant.
In Melandrium Y- chromosome is long as compare to X- chromosome.
In plant sex chromosomes are found only in unisexual plant.
Pro. R.P. Roy gave the importance of Y-chromosome in plant.
He discovered sex determination in Coccinea indica (Family- cucurbitaceae)
Y- chromosome contains four regions and X- chromosome contains two regions. Different functions of these

  1. I st region - (Female suppressor region) :- This region suppresses the development of female reproductive structures.
  2. II nd region (Male promotor region) :- This region initiates or start the development of Anther
  3. III rd region (Male fertility region) :- This region induces the further development of Anther.
  4. IV th region (Homologous region) :- This region helps in the disjunction & Pairing of X and Y chromosome during meiosis.
  5. V th region (Differential region of X-chromosome) :- This region induces the development of female gonads

So when one or more than one Y- chromosome present then plant is male and in female plant Ychromosome
is absent.

Special Case :
If I st region of Y chromosome is removed then plant becomes bisexual (XY).

If II nd region of Y chromosome is removed then plant becomes female due to absence of II nd region, I st region of Y chromosome does not suppress the V th region of X-chromosome.

If III rd region of Y chromosome is removed then plant become sterile male due to absence of III rd region so further development of anther does not take place.


Diploid organisms such as pea and Drosophila, have two alleles for each gene on each chromosome (the exceptions are for the X linked genes in XY or XO males). With the result, the recessive allele is not expressed
in the phenotype in presence of the dominant one. However, this is not so in the case of haploid organisms. Contrary to diploid organisms, the genetics of haploid organisms exhibit the following features:

1. Haploid organisms contain only one allele of a gene, so there is no complication of dominance. All the
genes, whether dominant or recessive, expresses itself in the offsprings.

2. In absence of dominance, any new mutation is immediately expressed in the phenotype, in haploid

3. Study of inheritance of the mutated gene, its linkage, crossing over and biochemical consequence of a
mutation can easily be studied in haploid.

Linkage And Recombination in Neurospora (Drosophila of plant kindgom)
Detection of linkage and recombination of genes in haploid organisms as in fungi, bacteria etc. is comparatively
simple. Fungus Neurospora is one of the favourite material with geneticists, because :-
1. The life cycle of Neurospora is the product of a single meiosis.
2. The life cycle is of a short duration.
3. The meiotic products are linearly arranged in ascus as 8 ascospores as ordered tetrads (i.e, the eight
ascospores are arranged in the same order in which chromatids were on the meiotic metaphase plate).

Tetrad Analysis in Ordered Tetrads –
In Neurospora, the nuclei from hyphae of opposite mating type (+) and (–) fuse to form a diploid zygote. The
zygote is the only diploid stage in the life cycle of Neurospora. The zygote nucleus divides meiotically producing four haploid nuclei, each of which then undergoes mitosis. The eight cells produced this way, form 8 haploid ascospores enclosed in the ascus. The three divisions proceed along the longitudinal axis, so the ascospores are arranged in a line in a specific order that indicates the order of arrangement of chromatids on the meiotic metaphase plate. This is called linear or ordered tetrad. Each of the four products of meiosis can be cultured separately to study their phenotypes and genotypes. This is called tetrad analysis.

1. First Division Segregation Between Centromere and gene-a.
A cross between two strain of Neurospora, one normal (a + ) and other mutant (a) strain produces 8-ascospores, out of which four are normal (a + ) and other four mutants (a). The linear arrangement of ascospores in ascus is 4a + : 4a. It indicates the absence of crossing over between locus-a and centromere. This is described as first divisionsegregation.

2. Second Division Segregation Between Centromere and Gene-a.
In a similar cross if crossing over takes place leading to paired arrangement of ascospores with a particular
gene, it is described as second division crossing over. The arrangement of ascospores in the sequence ( 2 :
2: 2 : 2) is as follows:

(i) a + : a + : a : a : a : a : a + : a +
(ii) a : a : a + : a + : a + : a + : a : a
(iii) a + : a + : a : a : a + : a + : a : a

Single Gene Mapping in Neurospora
In Neurospora centromere behaves as a gene for mapping gene pair. In such a case distance of gene from the
centromere is calculated by calculating the percentage of cross overs between centromere and gene.
Que. If 10% asci show crossing over in ascocarp what will be distance between gene and centromere.
If total 100 asci are present in a Neurospora

asci is derivative of 4 chromatids
100 asci are derivative of 400 chromatids = total chromatids
10 asci are derivative of 40 chromatids
(Out of 40 only 20 will be the recombinant type)


AP Biology Practice Test: Unit 5 — Heredity

The correct answer is (B). First define the variables and then create a chart:

Inflated Pod Shape = I
Constricted Pod Shape = i
Green Pod Color = G
Yellow Pod Color = g
Cross: IiGg x IiGg

Inflated and Green: 9/16
Inflated and Yellow: 3/16
Constricted and Green: 3/16
Constricted and Yellow: 1/16

How many different gametes could be made from a pea plant heterozygous for flower color, flower position and stem length?

Purple Flower Color = P
White Flower Color = p
Axial Flower Position = A
Terminal Flower Position = a
Tall Stem Length = T
Dwarf Stem Length = t

Gametes are as follows: PAT, PAt, PaT, Pat, pAT, pAt, paT, pat

This gene is an example of:

Sexual reproduction produces variation within populations through which mechanism(s)?

Questions 14–15

Lidicker and McCollum (1997) examined genetic variation in two populations of sea otters in the eastern Pacific. Before fur hunting led to their near extinction, sea otters were distributed throughout the region. Along the central California coast, it is estimated that only one population of 50 or fewer individuals survived. Since this population was protected in 1911, it has grown to over 1500 otters. The population may have lost considerable genetic variation due to the extreme reduction in population size. A population from Alaska experienced a similar bottleneck around that time, but it was not as severe.

One way to look at genetic diversity is to study the allele and genotype frequencies of allozymes. Allozymes are enzymes that show different rates of movement in gel electrophoresis due to the presence of different alleles at a single locus, whereas F is the fast-moving allele, and S is the slow-moving allele. The table below shows the number of individuals with a given genotype for six variable (polymorphic) two-allele loci:

We can use these data to calculate the allelic frequencies for a given locus, such as the ME locus in the California population. How many S alleles are there in the population?

Lidicker and McCollum (1997) examined genetic variation in two populations of sea otters in the eastern Pacific. Before fur hunting led to their near extinction, sea otters were distributed throughout the region. Along the central California coast, it is estimated that only one population of 50 or fewer individuals survived. Since this population was protected in 1911, it has grown to over 1500 otters. The population may have lost considerable genetic variation due to the extreme reduction in population size. A population from Alaska experienced a similar bottleneck around that time, but it was not as severe.

One way to look at genetic diversity is to study the allele and genotype frequencies of allozymes. Allozymes are enzymes that show different rates of movement in gel electrophoresis due to the presence of different alleles at a single locus, whereas F is the fast-moving allele, and S is the slow-moving allele. The table below shows the number of individuals with a given genotype for six variable (polymorphic) two-allele loci:

Watch the video: Module 6 Narrated Lecture through (June 2022).


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