Chapter 2Transmission Genetics: Heritage from Mendel

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Gregor Mendel• G. Mendel carried out his experiments from 1856 to

1863 in a small garden plot nestled in a corner of the St. Thomas monastery in the town of Brno

• He published the results and his interpretation in its scientific journal in 1866

• Mendel’s paper contains the first clear exposition of the statistical rules governing the transmission of hereditary elements from generation to generation

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Mendel’s Genetic Hypothesis

• Each parent contributes to its progeny distinct elements of heredity = factors = genes

• Factors remain unchanged as they pass through generations

• Mendel thought in quantitative, numerical terms. He looked for statistical regularities in the outcome from his crosses

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Mendel’s Experiments• Experimental organism: garden pea, Pisum

sativum

• Advantages: many known varieties with different alternative traits, self-fertilization, easy artificial fertilization

• True-breading varieties = self-fertilized plants produce only progeny like themselves

Figure 2.1: Crossing pea plants

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Figure 2.2: Reciprocal crosses of truebreeding pea plants

Figure 2.3: The seven character differences in peas studied by Mendel

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• True-breading plants with different forms of a trait, such as round vs. wrinkled seeds

• All of the F1 progeny exhibited only one parental trait (round seeds)

• In F2 generation obtained by self-fertilization of F1 plants, the observed ratio of visible traits was 3 round : 1 wrinkled

• Outcome of cross was independent of whether the trait came from the male or female parent: reciprocal crosses produced the same result

Mendel’s Experiments

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Figure 2.5: Expression of Mendel’s traits in plants and seeds

Table 2.1 Results of Mendel’s monohybrid experiments

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Mendel’s Hypothesis

• Each true-breading parent has two identical copies of the genetic information specifying the trait = homozygous

• Each gamete contains only one copy of a hereditary factor specifying each trait

• Random fertilization unites two copies of the gene in the progeny

• F1 progeny contains different variants (alleles) of the gene = heterozygous

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Mendel’s Hypothesis• The genetic constitution of an organism =

genotype• The observable properties of an organism =

phenotype• In the cross between round and wrinkled seed pea

plants:– Round seed parent has two identical copies of

genetic information = its genotype = AA– The genotype of a wrinkled seed parent = aa

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Dominance • Round seed parent contributes “A” gamete to

offspring• Wrinkled seed parent contributes “a” gamete to

offspring• Offspring genotype = A + a = Aa contains one copy of

“A” and one copy of “a”• All offspring produce round seeds although their

genotype is “Aa” because “A” is dominant and“a” is recessive

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Round vs. Wrinkled: Modern Context• The gene that determines the shape of a seed

encodes an enzyme, starch-branching enzyme I (SBEI), required to synthesize a branched-chain form of starch known as amylopectin

• Round (W) seeds contain amylopectin and shrink uniformly as they dry

• Wrinkled (w) seeds lack amylopectin and shrink irregularly

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Round vs. Wrinkled: Modern Context• Wrinkled peas have an inborn error in starch

metabolism

• The molecular basis of the wrinkled (w) mutation = SBEI gene is interrupted by the insertion of a DNA sequence called a transposable element

• Transposable elements = DNA sequences capable of moving (transposition) from one location to another

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Round vs. Wrinkled: Modern Context

• A procedure called gel electrophoresis is used to separate DNA molecules of different sizes

• DNA fragment corresponding to the W form of the SBEI gene moves farther than the w fragment, because the w fragment is larger (owing to the insertion of the transposable element)

17Figure 2.4: Banding as a result of distinct sizes of DNA molecules

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Round vs. Wrinkled: Modern Context• Classical geneticists studied primarily morphological

traits = the shape of a seed is manifestly round or wrinkled

• Modern geneticists study morphological traits, too, but they supplement this with molecular traits = the pattern of bands in a gel

• Morphological traits are frequently dominant or recessive, but this is not necessarily true of molecular traits

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• When alternative forms of a gene (W and w) can both be detected when they are present in the cell, we say that the forms of the gene are codominant

• Molecular traits are often (but by no means always) codominant

• Dominance is not an intrinsic feature of a gene; it rather depends on the method we chose to examine it

Round vs. Wrinkled: Modern Context

20Figure 2.6: A diagrammatic explanation of the 3 : 1 ratio of

dominant : recessive visible traits observed

Figure 20: Three attributes of phenotype affected by Mendel’s alleles W and w

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Segregation• When an F1 plant is self-fertilized, the A and a

determinants segregate from one another and are included in the gametes in equal numbers

• The gametes produced by segregation come together in pairs at random to yield the progeny of the next generation

• In the F2 generation, the ratio of the progeny with dominant trait to the progeny with recessive trait is 3:1. In case of round and wrinkle seeds, 3/4 round and 1/4 wrinkled offspring

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The Principle of Segregation

• The Principle of Segregation: • In the formation of gametes, the paired hereditary

determinants (genes) segregate in such a way that each gamete is equally likely to contain either member of the pair

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• Genetic cross : Aa X Aa produces A and a gametes from each parent

• Punnett square shows four possible outcomes = AA, Aa, aA, and aa

• Three combinations = AA, Aa, and aA produce plants with round seeds and display a round phenotype

• Fourth combination = aa displays wrinkled phenotype

Monohybrid Genetic Cross

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Figure 2.7: In the F2 generation, the ratio of WW : Ww : ww is 1 : 2 : 1.

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wrinkled

Monohybrid Genetic Cross

1/4AA

round dominant

1/2Aa

round dominant

1/4aa

recessive

Parents: Aa X AaEach parent produces A and a gametes and

contributes one gamete at fertilization

Figure 2.8: Mendel’s results of self-fertilization of the F2 progeny

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Testcross Analysis• Testcross = a cross between an organism of

dominant phenotype (genotype unknown) and an organism of recessive phenotype (genotype known to be homozygous recessive)

• In a testcross, the relative proportion of the different gametes produced by the heterozygous parent can be observed directly in the proportion of phenotypes of the progeny, because the recessive parent contributes only recessive alleles

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• AA + aa = Aa – testcross produces dominant progeny only: parent homozygous

• Aa + aa = 1/2 Aa + 1/2 aa – testcross produces 1/2 dominant and 1/2 recessive individuals: parent heterozygous

Testcross Results

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Figure 2.9: A testcross shows the result of segregation directly in the phenotypes of the progeny

Table 2.2 Results of Mendel’s testcross experiments

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Dihybrid Cross• Mendel studied inheritance of two different traits,

such as seed color (yellow vs. green) and seed shape (round vs. wrinkled) in the same cross = dihybrid cross

• The F1 progeny were hybrid for both characteristics, and the phenotype of the seeds was round (dominant to wrinkled) and yellow (dominant to green)

• In the F2 progeny, he observed the 9 round yellow : 3 wrinkled yellow : 3 round green : 1 wrinkled green ratio

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Dihybrid Cross• Mendel carried out similar experiments with

other combinations of traits. For each pair of traits, he consistently observed the 9:3:3:1 ratio

• He also deduced the biological reason for the observation:

• In the F2 progeny, if the 3:1 ratio of round: wrinkled is combined at random with the 3:1 ratio of yellow: green, it yields the 9:3:3:1 ratio of a dihybrid cross

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Figure 2.10: 9 : 3 : 3 : 1 ratio that Mendel observed in the F2 progeny of the dihybrid cross

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Independent Segregation• The Principle of

Independent Assortment: • Segregation of the

members of any pair of alleles is independent of the segregation of other pairs in the formation of reproductive cells.

Figure 2.11: Independent segregation of the Ww and Gg allele pairs

Figure 2.12: Diagram showing the basis for the 9 : 3 : 3 : 1 ratio of F2 phenotypes resulting from a cross

Figure 2.13: The ratio of homozygous dominant, heterozygous, and homozygous recessive genotypes

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Dihybrid Testcross • The progeny of testcrosses show the result of independent

assortment• The double heterozygotes produce four types of gametes in

equal proportions, the ww gg plants produce one type

• The progeny phenotypes are expected to consist of round yellow, round green, wrinkled yellow, and wrinkled green in a ratio of 1:1:1:1

This observation confirmed Mendel’s assumption that the gametesof a double heterozygote included all possible genotypes inapproximately equal proportions

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Figure 2.14: Genotypes and phenotypes resulting from a testcross of a Ww Gg double heterozygote

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Trihybrid Genetic Cross• Trihybrid cross = three pairs of elements that

assort independently, such as WwGgPp

• For any pair phenotypic ratio = 3:1

• For any two pairs ratio = 9:3:3:1

• Trihybrid cross pattern of segregation and independent assortment is identical to dihybrid

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Probabilities

• Addition Rule: The probability of the realization of one or the other of two mutually exclusive events, A or B, is the sum of their separate probabilities

• Mendelian patterns of inheritance follow laws of probability

• Prob {WW or Ww} = Prob {WW} + Prob{Ww} = 0.25 + 0.50 = 0.75

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Figure 2.15: The use of the addition and multiplication rules to determine the probabilities of genotypes and phenotypes

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Probabilities• Multiplication Rule:

The probability of two independent events, A and B, being realized simultaneously is given by the product of their separate probabilities

• Prob {WG} = Prob {W} x Prob{G} = 0.5 x 0.5 = 0.25

Figure 2.16: Two important types of independence

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Pedigree Analysis• In humans, pedigree analysis is used to determine

individual genotypes and to predict the mode of transmission of single gene traits

Figure 2.17: Conventional symbols used in depicting human pedigrees

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Autosomal Dominant• Huntington disease is a progressive nerve

degeneration, usually beginning about middle age, that results in severe physical and mental disability and ultimately in death

• The trait affects both sexes • Every affected person has an affected parent • ~1/2 the offspring of an affected individual are affected

Figure 2.18: Pedigree of a human family showing the inheritance of the dominant gene for Huntington disease

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• Albinism = absence of pigment in the skin, hair, and iris of the eyes

• The trait affects both sexes • Most affected persons have parents who are not

themselves affected; the parents are heterozygous for the recessive allele and are called carriers

• Approximately 1/4 of the children of carriers are affected

• The parents of affected individuals are often relatives

Autosomal Recessive

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Figure 2.19: Pedigree of albinism, a recessive genetic disorder

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Incomplete Dominance

• Incomplete dominance = the phenotype of the heterozygous genotype is intermediate between the phenotypes of the homozygous genotypes

• Incomplete dominance is often observed when the phenotype is quantitative rather than discrete

Figure 2.21: Incomplete dominance in the inheritance of flower color in snapdragons

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Multiple Alleles/Codominance• Codominance means that the heterozygous genotype

exhibits the traits associated with both homozygous genotypes

• Codominance is more frequent for molecular traits than for morphological traits

• Multiple alleles = presence in a population of more than two alleles of a gene

• ABO blood groups are specified by three alleles IA, IB and IO

• IA and IB codominant, both IA and IB are dominant to IO

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Multiple Alleles/Codominance

• People of: – blood type O make both anti-A and anti-B antibodies

– blood type A make anti-B antibodies

– blood type B make anti-A antibodies

– blood type AB make neither type of antibody

51Figure 2.22: The ABO antigens on the surface of human red blood cells are

carbohydrates

Table 2.3 Genetic control of the Human ABO Blood Groups

Figure 2.23: Antibodies against blood type antigens

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Expressivity and Penetrance

• Variation in the phenotypic expression of a particular genotype may happen because other genes modify the phenotype or because the biological processes that produce the phenotype are sensitive to environment

• Variable expressivity refers to genes that are expressed to different degrees in different organism

• Penetrance refers to the proportion of organisms whose phenotype matches their genotype for a given trait. A genotype that is always expressed has a penetrance of 100 percent

• A mutant gene is not always expressed in exactly in the same way

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Epistasis• Epistasis refers to any type of gene

interaction that results in the F2 dihybrid ratio of 9:3:3:1 being modified into some other ratio

• In a more general sense, it means that one gene is masking the expression of the other

• Flower color in peas: formation of the purple pigment requires the dominant allele of both the C and P genes: the F2 ratio is modified to 9 purple:7 white

Figure 2.24: A cross showing epistasis in the determination of flower color in peas

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Epistasis• There are nine possible dihybrid ratios when both

genes show complete dominance

Examples:• 9:7 occurs when a homozygous recessive mutation in

either or both of two different genes produces the same phenotype

• 12:3:1 results when a dominant allele of one gene masks the genotype of a different gene

• 9:3:4 is observed when homozygosity for a recessive allele masks the expression of a different gene

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Figure 2.25: Modified F2 dihybrid ratios. In each row, different colors indicate different phenotypes