12.2: Patterns of Inheritance - Biology

12.2:  Patterns of Inheritance - Biology

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12.2: Patterns of Inheritance

Chapter 8: Introduction to Patterns of Inheritance

Figure 8.1 Experimenting with thousands of garden peas, Mendel uncovered the fundamentals of genetics. (credit: modification of work by Jerry Kirkhart)

Genetics is the study of heredity. Johann Gregor Mendel set the framework for genetics long before chromosomes or genes had been identified, at a time when meiosis was not well understood. Mendel selected a simple biological system and conducted methodical, quantitative analyses using large sample sizes. Because of Mendel’s work, the fundamental principles of heredity were revealed. We now know that genes, carried on chromosomes, are the basic functional units of heredity with the ability to be replicated, expressed, or mutated. Today, the postulates put forth by Mendel form the basis of classical, or Mendelian, genetics. Not all genes are transmitted from parents to offspring according to Mendelian genetics, but Mendel’s experiments serve as an excellent starting point for thinking about inheritance.

Connection for AP® Courses

Connection for AP ® Courses

The characteristics that Mendel evaluated in his pea plants were each expressed as one of two versions, or traits, for example, green peas versus yellow peas). As we will explore in more detail in later chapters, the physical expression of characteristics is accomplished through the expression of genes—sequences of DNA—carried on chromosomes. The genetic makeup of peas consists of two similar, or homologous—remember this term from Chapter 11—copies of each chromosome, one from each parent. Through meiosis, diploid organisms utilize meiosis to produce haploid (1n) gametes that participate in fertilization. For cases in which a single gene controls a single characteristic, such as pea color, a diploid organism has genetic copies that may or may not encode the same version of the characteristic. These gene variations, for example, green peas versus yellow peas—are called alleles.

Different alleles for a given gene in a diploid organism interact to express physical characteristics such as pea color in plants or hairline appearance in humans. The observable traits of an organism are referred to as its phenotype. The organism’s underlying genetic makeup, that is, the combination of alleles, is called its genotype. When diploid organisms carry the same alleles for a given trait, they are said to be homozygous for the genotype when they carry different alleles, they are said to be heterozygous. For a gene whose expression is Mendelian (Section 12.1), homozygous dominant and heterozygous organisms will look identical, that is, they will have different genotypes but the same phenotype. The recessive allele will only be observed in homozygous recessive individuals.

However, alleles do not always behave in dominant and recessive patterns. In other words, there are exceptions to Mendel’s model of inheritance. For example, incomplete dominance describes situation in which the heterozygote exhibits a phenotype that is intermediate between the homozygous phenotypes (e.g., a pink-flowered offspring is produced from a cross between a red-flowered parent and a white-flowered parent). Codominance describes the simultaneous expression of both of the alleles in the heterozygote (e.g., human blood types). It is also common for more than two alleles of a gene to exist in a population (e.g., variations in sizes of pumpkins). In humans, as in many animals and some plants, females have two X chromosomes, and males have one X chromosome and one Y chromosome. Genes on the X chromosome are X-linked, and males inherit and express only one allele for the gene (e.g., hemophilia, color-blindness). Some alleles can also be lethal, so their phenotype will never be observed.

Many human genetic disorders, including albinism, cystic fibrosis, and Huntington’s disease can be explained on the basis of simple Mendelian inheritance patterns created by pedigree analysis. In later chapters, we will learn how DNA analysis can be used to diagnose genetic disorders. Punnett squares are useful tools that apply the rules of probability and meiosis to predict the possible outcomes of genetic crosses. Test crosses are done to determine whether or not an individual is homozygous or heterozygous by crossing the individual with a homozygous recessive.

Information presented and the examples highlighted in the section support concepts outlined in Big Idea 3 of the AP ® Biology Curriculum Framework. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP ® Biology course, an inquiry-based laboratory experience, instructional activities, and AP ® exam questions. A Learning Objective merges required content with one or more of the seven Science Practices.

Big Idea 3 Living systems store, retrieve, transmit, and respond to information essential to life processes.
Enduring Understanding 3.A Heritable information provides for continuity of life.
Essential Knowledge 3.A.3 The chromosomal basis of inheritance proposed by Mendel provides an understanding of the pattern of passage of genes from parent to offspring.
Science Practice 1.1 The student can create representations and models of natural or man-made phenomena and systems in the domain.
Science Practice 7.2 The student can connect concepts in and across domain(s) to generalize or extrapolate in and/or across enduring understandings and/or big ideas.
Learning Objective 3.12 The student is able to construct a representation (e.g., Punnett square) that connects the process of meiosis to the passage of traits from parent to offspring.
Essential Knowledge 3.A.3 The chromosomal basis of inheritance proposed by Mendel provides an understanding of the pattern of passage of genes from parent to offspring.
Science Practice 3.1 The student can pose scientific questions.
Learning Objective 3.13 The student is able to pose questions about ethical, social, or medical issues surrounding human genetic disorders.
Essential Knowledge 3.A.3 The chromosomal basis of inheritance proposed by Mendel provides an understanding of the pattern of passage of genes from parent to offspring.
Science Practice 2.2 The student can apply mathematical routines to quantities that describe natural phenomena.
Learning Objective 3.14 The student is able to apply mathematical routines to determine Mendelian patterns of inheritance provided by data sets.
Essential Knowledge 3.A.4 The inheritance patterns of many traits cannot be explained by simple Mendelian genetics.
Science Practice 6.5 The student can evaluate alternative scientific explanations.
Learning Objective 3.15 The student is able to explain deviations from Mendel’s model of the inheritance of traits.
Essential Knowledge 3.A.4 The inheritance patterns of many traits cannot be explained by simple Mendelian genetics.
Science Practice 6.3 The student can articulate the reasons that scientific explanations and theories are refined or replaced.
Learning Objective 3.16 The student is able to explain how the inheritance pattern of many traits cannot be accounted for by Mendelian genetics.
Essential Knowledge 3.A.4 The inheritance patterns of many traits cannot be explained by simple Mendelian genetics.
Science Practice 1.2 The student can describe representations and models of natural or man-made phenomena and systems in the domain.
Learning Objective 3.17 The student is able to describe representations of an appropriate example of inheritance patterns that cannot be explained by Mendel’s model of the inheritance of traits.

The Science Practices Assessment Ancillary contains additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards:

  • [APLO 3.12]
  • [APLO 3.14]
  • [APLO 3.16]
  • [APLO 3.11]
  • [APLO 3.13]
  • [APLO 3.17]

The seven characteristics that Mendel evaluated in his pea plants were each expressed as one of two versions, or traits. The physical expression of characteristics is accomplished through the expression of genes carried on chromosomes. The genetic makeup of peas consists of two similar or homologous copies of each chromosome, one from each parent. Each pair of homologous chromosomes has the same linear order of genes. In other words, peas are diploid organisms in that they have two copies of each chromosome. The same is true for many other plants and for virtually all animals. Diploid organisms utilize meiosis to produce haploid gametes, which contain one copy of each homologous chromosome that unite at fertilization to create a diploid zygote.

For cases in which a single gene controls a single characteristic, a diploid organism has two genetic copies that may or may not encode the same version of that characteristic. Gene variants that arise by mutation and exist at the same relative locations on homologous chromosomes are called alleles . Mendel examined the inheritance of genes with just two allele forms, but it is common to encounter more than two alleles for any given gene in a natural population.

Which of these best describes the inheritance pattern of skin color? multiple allele in which offspring receive three alleles from each parent multiple allele in which offspring receive three alleles from only one parent polygenic in which offspring receive three alleles from each parent polygenic in which offspring receive three alleles from only one parent

polygenic in which offspring receive three alleles from each parent.

Polygenic inheritance, the etymology itself, describes the trait of inheritance that is determined by more than one gene. It contradicts the Mendelian inheritance patterns which suggest that traits are determined by one gene.

The human skin color is a good example of polygenic or multiple-gene inheritance. The melanin pigment is responsible for the dark coloration of the skin and it must be noted that there are at least three genes which control the human skin color.

Utilizing a hypothetical example where the production of melanin is controlled by a so called contributing alleles (denoted as A, B, C) would result in a dark skin color and non-contributing alleles produce light skin color, it is not impossible to see a spectrum of skin colors would result in an offspring.

It must be taken into consideration that alleles do not display dominance over the other alleles. But rather, the contributing alleles does not mask the effects. It gives an additive effect- this means that each contributing allele produces one color unit.

Using two parents, heterozygous (two alleles of different types each) for each melanin-producing genes, (AaBbCc X AaBbCc), it is possible to see how additive effects and alleles combination would result to a varying types of genotypes.

Therefore, it needs an interaction between several different genes and for this to occur, it needs three alleles from both of its parents.

The answer would be, -polygenic in which offspring receive three alleles from each parent.

12.2: Patterns of Inheritance - Biology

Brother Joseph looked, and saw several sheets of the monastery's best note paper spread out on Mendel's desk. On each sheet his friend had written, in neat Germanic handwriting, columns of figures, interspersed with occasional blotches of ink from a faulty pen.

"What are they?" Brother Joseph asked.

"Last year's results on Pisum hybrids," he was told, "I'm doing the numbers. This column is the experiment number, the next is the number of fertilizations, this is the number of plants, and here we see the color of pod results in the first filial generation. I'm calling it the F1 generation."

the word 'filius' means 'son' in Latin Brother Joseph looked over his friend's shoulder and ran his finger down the last column, reading off some of the numbers. "428, 152, 580, 73.79 percent, 26.21 percent - " his voice trailed off. "What does this all mean?"

Mendel looked up at him and rubbed more ink into his face. "You are looking at some of the F2 results," he said, shuffling more papers. "Here are the F1's"

"Wait, wait," Brother Joseph laughed, "wait a moment, I'm getting lost. What are all these numbers and what do they mean?"

raw data often has to be interpreted, Mendel did this very well "Well," said Mendel slowly, "they are only preliminary, and will have to be repeated, but these are the results of my first experiments into plant hybridization and a possible mechanism for inheritance."

"Using your ideas about small particulate " transmission elements " that control the development of the plant body and form?"

"Yes. If I'm right, these numbers confirm that each parent plant in a genetic cross, has . but why am I telling you all this? We need more results before I can be sure. We also need to test more factors and more traits to see if they all behave in the same way."

"You want us to do more crosses, and gather more data?"

well tested starting material is important in these experiments "Yes, that would be excellent. I've started some hybrids already. These peas," and he picked up a bag from his desk, "are 'pure breeding' forms that I have extensively tested over the last several years. They always give consistent results and they will be our starting material. These," and here he picked up a second bag of seeds, "are the seeds of an F1 hybrid. At least part of our work must be to cross these with at least three types of other plants."

"Three?" Brother Joseph asked, "why three other types?"

this is new
no one had considered this idea before

Page Items * The Story
* Combinations
* The Question
* Start the work
* Interpretation
"Because it is not always obvious by looking at the form, the soma , of a plant, which transmission elements it contains. It is frustrating, but at least two kinds of plants, both of which look the same and have the same outward form of a trait, may hold different combinations of transmission elements."

"How can that be? If these 'elements' control the form of a trait, shouldn't you be able to tell what 'elements' are inside a plant by what it looks like on the outside?"

"You might think so, and some of the time you can, but I suspect that there may be cases where that would be impossible."

"More difficult, but not impossible. Let's do the experiments and find out!"

the start of the investigation

Page Items * The Story
* Combinations
* The Question
* Start the work
* Interpretation

- what are the patterns, and what do they mean?

B rother Gregory wants you to investigate the patterns of inheritance seen in his pea hybrids as the traits are inherited through two generations (called the F1 and F2 generations).

If he is right, and the form of a trait is controled by a 'transmission element', an offspring recieves one 'element' from its male parent and a second 'element' from its female parent.

Once in the body of the offspring, these 'elements' direct the development of the traits they control. It should be possibile to determin what 'elements' each offspring inherits by the numbers, and ratios, of the offspring showing those traits.

    two parent plants that are 'pure breeding',

the question What different patterns of inheritance can be seen during genetic crosses, and how can these patterns be interpreted?

Plant Hybridization
---click here to start the simulation ---

First Genetic Cross - to produce F1 hybrids

Select " pure breeding tall plant " from the Special Peas menu. This will become " TRAIT ONE " of Parent One (or Parent Two).

Select " short plants " from the Traits menu. Click on " TRAIT ONE " of the other Parent.

You should now have two parent plants. You know that the 'tall' plant is 'pure breeding', but what do you know about the 'short' plant? Is it 'pure breeding'? How would you know? (Hint: haven't you checked this already?).

Carry out the genetic cross by clicking on the " Collect Peas " box, collecting the seeds and then clicking on the " Plant Peas " button. The new peas will grow and number and type of offspring will appear in the boxes underneath.

record your results Write down, and record

Repeat this type genetic cross experiment several times, and then use the other 'special pea' that is 'pure breeding for the purple flowered plant'. The other parent in this cross should hold the 'white floweres' trait.

This is now the data for the 'pattern of inheritance' seen as Mendel's transmission elements are passed from the original parent plants into the first generation of hybrids, the F1 hybrids.

Second Genetic Cross - to produce F2 hybrids

genetic crosses involving the F1 hybrids One of the F1 hybrid plants (produced in the first round of genetic crosses) must be one of the parents in the second round of genetic crosses.

To do this, click on the 'special pea' called " an F1 seed from a tall/short cross " and this version of the trait will become one of the parents.

    another F1 hybrid plant from a tall/short cross . To do this click again on that 'special seed'. Both parents should now be these F1 hybrids.

Repeat these genetic cross many times (at least 10 times each). Record all the results.

answer these questions What did you find? Which of the 'F1 crosses' produced consistent results? Which of the "F1 crosses' produced inconsistent results? How do you explain your results?

calculation of ratios The Raw Data

Brother Gregory was able to make sense of his raw data because of the way he interpreted the relationship between the sets of numbers.

In one of his famous experiments he obtained the following results for a cross of two F1 plants to give the F2 offspring:

What does this raw data mean?

Page Items * The Story
* Combinations
* The Question
* Start the work
* Interpretation
Interpretation -

    Percentages : Mendel calculated the percent of his F2 plants that were tall, e.g.

percent tall = 787/1064 x 100 = 73.96%

When interpreted this way, the variation in the raw numbers seen from one experiment to the next, suddenly vanishes! In the F2 generation, the percentage of tall plants (and the percentage of short plants) becomes constant (or almost so), and the ratio of one version of the trait to the other version also becomes almost constant!!

Analyze and Interpret Your Data

For all of the genetic crosses you have carried out in this investigation, calculate the percentages of offspring that show one trait or the other, and also calculate the ratios of one trait to another.

12.2: Patterns of Inheritance - Biology







Mendelian inheritance is based on the transmission of a single gene on a dominant, recessive or X-linked pattern. Discoveries on DNA structure, the genetic code, the genome and the observation that some characters and hereditary diseases do not follow classical mendelian inheritance have led researchers to define other patterns of transmission, referring particularly to multifactorial and mitochondrial inheritance. Multifactorial inheritance is based on the synergy of genes and environmental factors. Extra nuclear mitochondrial heredity can only be transmitted by the mother whose cells contain a number of mitochondria. Several factors can modify the expected individual phenotypes.There will undoubtedly be important advances in our knowledge of the pattern of inheritance of characters and diseases given a better understanding of gene structure and role, interaction of genes between them and with the environment.

  • Autosomal dominant inheritance
  • Autosomal recessive inheritance
  • X linked chromosome recessive inheritance
  • An eukaryote gene is made of successive coding segments (exons) and not coding ( introns) ---> pre-messenger RNA splicing ---> RNA messenger
  • Meiosis ( one diploid cell with 46 chromosomes ---> 4 haploid cells with 23 chromosomes) is, with mutations, responsible for diversity and genetic mixing by:
    • random dispersion of chromosomes in the gametes
    • exchange between homologous chromosomes ( crossing over).
    • either modify the phenotype: this is expressed as a dominant pattern (D)
    • or not modify the phenotype: recessive gene (R)
    • One individual who at the same locus has 2 identical alleles is known as homozygous (HOZ) for this allele.
    • One individual who has 2 different alleles at the same locus is called heterozygous (HEZ)for this allele.
      ---> a recessive character is phenotypically expressed only in the HOZ state.

    I.2 Autosomal dominant inheritance (AD):

    Most frequent instance: Aa x AA ( marriage of an affected individual HEZ with a normal individual).

    • Affected individuals are always the product of a parent carrier of the same character (except in a mutation).
    • The character is apparent in each generation ( does not skip a generation, except when the penetrance is reduced).
    • There are as many daughters and sons affected.
    • In a sibship one finds as many affected as normal individuals.
    • Half of descents of an affected individual will be affected.
    • All children of a normal individual will be normal.
    • Consanguinity is not elevated.
    • The character can be expressed if there is a mutation and be transmitted or eliminated if the defect is severe.
    • Most of the time one ignores what would be a HOZ individual for a dominant character.
    • Some observations suggest that the individual would be affected earlier and more severely or that the disease would progress more rapidly.
    • Penetrance and expressivity play a role.
    • If a disease is not compatible with reproduction, its frequency equals the mutation rate.
    • Achondroplasia
    • Aniridia
    • Marfan syndrome
    • Steinert myotonic dystrophy
    • Polydactyly
    • Adenomatosous polyposis of the colon

    I.3 Autosomal recessive inheritance

    Most frequent case: Aa x Aa (marriage of 2 normal heterozygotes).

    Parental genotype: Aa x Aa

    • In the instance of a rare disease, affected individuals have normal parents.
    • There are as many daughters and sons affected.
    • In a sibship there are usually one affected and three normal individuals.
    • An affected individual who marries a normal, non consanguineous person, usually has normal children.
    • However the disease can affect only one individual who has mutant genes: due to the small number of sibs in families this does not mean that this situation is due to a de novo mutation.
    • The frequency of consanguineous families is elevated ( the risk of matings between 2 individuals, carriers of the same mutation, and with common ancestors is increased ) and more so if the disease is rare.
    • When a mutation occurs it will not be apparent in the individual carrier
    • Mating between HOZ individuals is a common event: individuals with similar handicaps ( deafness, vision defect. ) often attend the same medical clinics and social functions which may facilitate the establishment of relationships.
    • Most enzyme deficiency diseases are autosomal recessive.
    • In fact few genes are completely recessive and often we can detect HEZ carriers.
    • HEZ is different of the two types of HOZ: intermediary inheritance the ability to detect HEZ allows for genetic counseling.
    • Often affected HOZ die early or do not reproduce.
    • Sometimes the affected HOZ will survive and reproduce ( ex: albinism) . If this affected individual marries a HEZ individual with a normal phenotype, the pattern of inheritance will appear incorrectly as a dominant transmission.
    • Even if the disease is rare, HEZ frequency may be elevated ( cystic fibrosis incidence 4/10,000. ---> heterozygotes frequency: 4/100).
    • Penetrance and expressivity ought to be considered.

    • Glycogenosis, VI types.
    • Sugar intolerance: galactose, fructose, saccharose, lactose.
    • Mucopolysaccharidoses VI types, except Hunter disease MPS II which is RLX.
    • Most amino acid disorders : phenylketonuria, tyrosinosis, cystinosis, leucinosis. albinism variants (except ocular albinism which is RLX) etc…
    • Several lipid metabolism diseases.
    • Wilson disease.
    • Several disorders of hormono synthesis, mainly thyroid and adrenal.
    • Sickle cell anemia, Thalassemia.
    • Factor I,II,V,VII,XII,XIII deficiencies
    • Cystic fibrosis

    I.4 X linked recessive inheritance (RLX)

    I.4.1 Most frequent case: heterozygote woman, a normal carrier who marries a normal man.

    • Affected individuals are usually born of normal parents.
    • In the paternal progeny all individuals are normal.
    • In the maternal progeny one often finds affected brothers or male sibs.
    • Usually affected individuals are male.
    • In the affected sibship , one male out of two is affected, and one female out of two is carrier.

    I.4.3 Marriage HEZ female / affected male

    Situation not likely to happen for a rare gene, but more frequent if the gene frequency is high ( ex: colour blindness).

    ---> The fact that the disease is restricted to males is not an absolute criterion of X linked inheritance. The criterion of n on transmission from father to son is more objective

    ---> (it allows to differentiate between autosomal dominant diseases with sex limitation).

    Remarks: to detect heterozygote carriers for genetic counseling.

    • Colour blindness
    • Hemophilia A and B
    • Angiokeratosis (Fabry disease)
    • Duchenne muscular dystrophy
    • Incontinentia pigmentosum
    • Agammaglobulinemia, Bruton type
    • G6PD deficiency

    I.5 Factors affecting the phenotype

    • The same mutation can induce different phenotypes.
    • Some diseases are due to a mutant gene with a variable structure then susceptible to produce different phenotype effects. In cystic fibrosis there are several mutations at the locus of gene CFTR. More especially in this disease we find patients mainly affected with pulmonary disease, pancreatic insufficiency and / or intestinal disease.

    Infrequently homologous chromosomes can have an uniparental origin. This is called a maternal or paternal disomy for a pair of homologous chromosomes. For example an individual affected with cystic fibrosis had one parent carrier of a known mutation for which he was homozygous having received two chromosomes 7 from the same parent carrier of this mutation and none from the other. Disomies are rare and their effect is not well known yet.

    1.5.7 Imprinting / parental sex influence

    During the course of development maternal and paternal genomes are not equivalent but complementary due to an epigenetic phenomenon that occurred during gametogenesis

    Gene function can vary depending upon the maternal or paternal origin of the allele in question.

    - A deletion on chromosome 15 ( 15q11-13) in the paternal chromosomal complement will lead to a Prader Willi syndrome different from the Angelman syndrome observed if the deletion involves the maternal chromosome.

    • In myotonic dystrophy the disease will be more severe, even often congenital, if the mother is the affected parent who transmitted the disease.
    • In Huntington disease the age of onset may be earlier and the severity more pronounced if the father transmitted the disease.

    --->Fertilization of an ovum without a nucleus by a sperm cell that has undergone a duplication of its haploid set or a dispermy could lead to a hydatiform mole.

    1.5.8 Gene interaction / Co-factors

    • For some individuals rickets is due to a vitamin D deficiency that will be corrected by the addition of a vitamin supplement in the diet. For others the disease due to the absence of the active form of Vitamin D, an autosomal recessive disease, or several other mutations regulating the vitamin D metabolism.
    • Mutations of cancer suppressors, protein regulators ( enzymes) or DNA repair genes have been identified. For example those mutations can induce metabolic diseases like mucopolysaccharidoses, ovary and colon cancers and DNA repair defects like Ataxia telangiectasia.

    1.5.9 Genes susceptibility to cancer and malformations

    • If a deletion occurs in gene WT1 located on chromosome 11 in region 11p13, it will lead to a Wilms tumor and a nephropathy. Syndrome WAGR ( W:: Wilms A: aniridia, G: genito-urinary malformations, R: mental retardation) would result from the deletion of this and other contiguous genes located in region 11p13-11p14.
    • Another example is Beckwith-Wiedemann syndrome also located on chromosome 11, in region 11p15.5, that would imply several contiguous genes. The syndrome manifests with obesity, macroglossia, nephroblastoma ( hepatoblastoma, neuroblastoma) gigantism and omphalocele. Growth factor IGF-2 (‘insulin-like growth factor 2’) a paternally expressed gene would be responsible for the pathogenesis of macrosomia mutation of other genes like CDKNIC ( p57K1P2) ( maternal expression), may be responsible for other aspects of the phenotype.

    ---> Other case reports and molecular studies are essential to circumvene etiological mechanisms in those malformation syndromes with susceptibility to cancer.

    1.5.10 Paternity

    A false paternity may sometimes be at the origin of an incomplete or incorrect family history. Doubt may arise about the paternity of an individual if ongoing molecular studies do not find in the suspected father the presence of one or more DNA sequences.

    1.5.11 Diagnostic error / classification

    Difficulties encountered sometime in the evaluation of a pattern of transmission of a disease may be due to diagnostic or classification errors. Several groups of diseases like glycogenoses and mucopolysaccharidoses often have a similar phenotype but a different enzymatic deficiency confirmed by the identification of a specific mutation for each one of them.


    • Multiallelic : there are at the same locus several possible alleles each individual has only 2 and the transmission is done on a monogenic mode.
    • Multifactorial:
      • There are for one specific character, a series of genes (and not loci) that form the basis of its identity ( synonymous : polygenic system, quantitative inheritance, quantitative heredity, multiple factors).
      • Their study is mathematical and complex.
      • Contributing role of environmental factors. Examples: height of the individual, cardiopathies, epilepsy.

      1. Continuous quantitative heredity

      Distribution of the population on a Gauss model curve ( ex: height).
      Threshold model often arbitrary.

      2. Discontinued quantitative heredity

        • Quite often certain characters have a discontinued binary distribution, meaning that they are present or not in an individual ( club foot, cleft palate, pyloric stenosis, diabetes, congenital cardiopathies, etc.. ) but their inheritance is as if they were multifactorial characters this is due to a threshold effect that makes them appear as discontinued: multifactorial inheritance with a threshold.
        • The individuals related to the population at the right of the threshold have a higher risk of being affected, and this is more so if they are closely related ( if p is the frequency of a polygenic character in the population, the risk for first degree related individuals is approximately the square root of p).
        • The threshold can be different for men and women for some diseases ( pyloric stenosis: boys are 5 times more often affected than girls : congenital dislocation of the hip approximately 7 times more frequent in women than men)
        • The risk of recurrence is higher when the first affected newborn is of the least susceptible gender of being affected.
        • The disease is more frequent in individuals related to the patient than it is rare in the general population
        • The more severe is the expressivity of a disease and the more elevated is the risk of recurrence.
        • The risk of recurrence increases with the number of affected individuals in the progeny.
        • Consanguinity plays a role: an individual whose spouse is related to him (her) has a higher risk of bringing together exact copies of the deleterious genes that were responsible for a given malformation in the family than if he was to marry to an individual chosen at random in the population. (If a couple has a child affected with an autosomal recessive disease, the risk of the next child of being affected is the same, irrespective of the parents being related or not, that is 1/4. If the risk is more elevated when the parents are related than when they are not, heredity is said to be polygenic).
        • Cleft palate
        • Hare lip and cleft palate
        • Cardio-vascular disesases
        • Schizophrenia
        • Diabetes
        • Gout
        • Hip dislocation
        • Strabismus
        • Psoriasis etc…etc…


        Mitochondrias come from ancestor anareobic bacterias ---> they have their own DNA. We then have extranuclear DNA in our cells.

        • Circular DNA of 16 kb for which the sequence is entirely known.
        • 37 genes code for 13 proteins, ribosomal RNA and transfer RNA.
        • The genetic code is different from the universal code (1): Mito Univ UGA Trp STOP AUA Met Ile AGA/AGG STOP Arg.
        • Mitochondria are present in the ovocyte (in large number).
        • ---> non mendelian inheritance: strictly maternal inheritance.
        • There are hereditary diseases due to mutant mitochondrial genes.
        • Mitochondrial cytopathies are often deleterious with a pleiotropic symptomatology ( multiple), since the deficit involves several organs: Pearson syndrome: exocrine pancreatic insufficiency, medullar insufficiency/ myelodysplasia, muscular deficit, hepatic, renal and gastro intestinal diseases.
        1. solely by women.
        2. to all her descents. Often the genetic defect is not present in all-but in a fraction only of mitochondria transmitted to the next generation then according to the number of gene mutations in mitochondria.
        3. variable expressivity.

        The term mitochondrial cytopathy may be ambiguous : the mitochondrial cytopathies include not only the pathologies due to mitochondrial gene mutations but also those due to nuclear genes coding for proteins invoved in the mitochondrial metabolism (enzymes of the respiratoiry chain).

        • Leber optic atrophy
        • Mitochondrial myopathies
        • Pearson syndrome…

        Pattern of Inheritance

        • This probability is the same for every pregnancy, regardless of the number of children the parents have. In addition, there is a so-called horizontal transmission wherein siblings born in the same generation are affected.
        • Most of the time, the heritable condition has an early onset with a more severe phenotype.
        • Furthermore, since the gene is located in the autosomes, males and females have the same chances of being affected.
        • The diagram below will help explain the possibilities of acquiring the gene from the parents. Autosomal Recessive (Image Credit: Wikimedia)

        The concept of a compound heterozygote is very crucial to the understanding of autosomal recessive inheritance. This concept refers to the fact than an affected individual bear two alternative forms of a single gene (referred to as an allele). This is very different from the concept of homozygous mutations where both forms of a particular gene exhibit identical forms of the mutation.

        Further information regarding autosomal recessive inheritance can be obtained from this video link:

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        Just select your click then download button, and complete an offer to start downloading the ebook. If there is a survey it only takes 5 minutes, try any survey which works for you.

        Watch the video: - Modes of Inheritance (June 2022).


  1. Kasia

    YES, it is exact

  2. Guiseppe

    In my opinion it is obvious. Try to look for the answer to your question in

  3. Rechavia

    It is happiness!

  4. Devland

    It happens. Let's discuss this issue. Here or at PM.

  5. Stearn

    Happy New Year to you and all readers!

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