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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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Each parent has two copies of the genetic information=homozygous

Each gamete contains only one copy of the genetic information

Random fertilization unites one copy from each parent

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

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Dominance:

All F1 offspring produce round seedsalthough their genotype is “Ww” because “W” is dominant and “w” is recessive

The genotypic ratio of the F2 is

1/4 WW

2/4Ww

1/4ww

The phenotypic ratio of the F2 is

3/4 Smooth

1/4 Wrinkled

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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 have a mutation in the starch-branching enzyme I (SBEI) and cannot make amylopectin and shrink irregularly

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Monohybrid Genetic Cross

Both F1 parents are heterozygous for the

gene.

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• If the unknown is WW, all the progeny of the testcross will be Ww (dominant) progeny

• If the unknown is Ww, 1/2 the progeny of the testcross will be Ww (dominant) and 1/2 the progeny will be ww (recessive)… As in the case shown on the right.

Testcross Analysis

A method to figure out the genotype of the organism… Is this pea WW or Ww?

We cross this organism with one we know is homozygous recessive. Then, the phenotypes of the can be attributed solely to the gene they inherited from the Unknown parent.

?

?

unknown

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Dihybrid Cross• two different traits, such as seed color (yellow vs. green) and seed

shape (round vs. wrinkled) in the same cross = dihybrid cross• Breed F1 that you know are hybrid for both characteristics

WG Wg wG wg

WG WWGG WWGg WwGG WwGg

Wg WWGg WWgg WwGg Wwgg

wG WwGG WwGg wwGG wwGg

wg WwGg Wwgg wwGg wwgg

Phenotypic ratio of 9/16 : 3/16 : 3/16 : 1/16

W w

W WW Ww

w Ww ww

G g

G GG Gg

g Gg gg

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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.

Fig. 2.11

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ProbabilitiesMultiplication Rule: The probability that two independent events, A and B, are realized

simultaneously is given by the product of their separate probabilities

Addition Rule: The probability that one or the other of two mutually exclusive events,

A or B, is the sum of their separate probabilities

What fraction would we expect to be

Round AND Green 3/4 x 1/4 = 3/16

What fraction would we expect to be

(Round and Green) OR (wrinkled and yellow) 3/16 + 3/16 =

6/16

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Pertubations on the theme

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

• Codominance means that the heterozygous genotype exhibits the traits associated with both homozygous genotypes

• 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

• Epistasis refers to a gene interaction that results in a modified F2 dihybrid ratio other than 9 : 3 : 3 : 1. One gene essentially masks the expression of the other.

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

MutS and colon cancer

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Epistasis

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Complementation reveals whether two recessive mutations are alleles of different genes

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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

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Autosomal Dominant Traits• Huntington disease is a progressive nerve degeneration, usually beginning

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

• Every affected person has an affected parent • ~1/2 the offspring of an affected individual are affected

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

Autosomal Recessive Traits

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How is this trait most likely inherited?

If individual III5 and III6 have another child,

what’s the probability that the child will be affected?

If individual III1 and III7 were to have a child,

what’s the probability that the child will be affected?

Both individuals are carriers (heterozygous) for the recessive allele, so there is 1/4 chance

the next child will be affected.

IF both individuals are heterozygous for the recessive allele, there is 1/4 chance the next

child will be affected.

There is a 1/2 chance III1 is a carrier AND a 1/2 chance that III7 is a carrier AND a 1/4 chance that their child would be affected if

they are carriers.

1/2 X 1/2 X 1/4 = 1/16

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How is this trait most likely inherited?

If individual III4 and III6 have a child, what’s the probability that the child will be affected?

Zero

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If the “nontaster”

allele is relatively

common in the

population, how is this trait most

likely inherited?

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How is this trait most likely inherited?