CO 03 Extension to Mendel: complexities in relating genotype to phenotype.

CO 03

Extension to Mendel: complexities in relating genotype to phenotype

1. Single-gene inheritance :

a. deviation from complete dominance and recessiveness.

b. Multiple alleles

c. One gene determine more than one trait

2. Multifactorial inheritance

Extension to Mendel

Fig. 3.2

Different dominant relationship

Fig. 3.3

Pink flower are the

result of imcomplete

dominance

Fig. 3.4

In codominance, F1 hybrid display the traits of both parents

spotted dotted



b. Multiple alleles

c. Recessive lethal allele

d. One gene determine more than one trait


Extension to Mendel

Fig. 3.5

ABO blood type are determined by three alleles of one gene

Fig. 3.6

How to establish thedominance relationsbetween multiple alleles

Mutations are the source of new alleles

Wild-type allele: frequency more than 1%

Mutant allele: frequency less than 1%

Monomorphic(One wild-type allele)

ABO blood type: polymorphic

agouti

black/yellowblack



b. Multiple alleles




Extension to Mendel

Two alleles with recessive lethal

Some alleles may

cause lethality

Table 3.1



b. Multiple alleles



Extension to Mendel

A single gene determines a number of distinct and seemingly

unrelated characteristics is known as pleiotropy.

Sickle-cell anemia

Mutant -globin aggregates to form long-fiber

Pleiotropy

Pleiotropy of sickle-cell anemia: dominance relation vary

Cells break down

Oxygen drops

Cells break down before malarial has a chance to reproduce



b. Multiple alleles



a. Two genes can interact to determine one trait

b. Heterogeneous trait

c. The same genotype does not always produce the same phenotype

Extension to Mendel

Fig. 3.11

How two genes interact to produce novel phenotypes

9:3:3:1, four distinct phenotypes, dihybrid cross of two independent assortment genes

F2 self cross

Fig. 3.12

Complementary gene action

One dominant allele of each of two genes is necessary to producethe phenotypes.

Fig. 3.13

Epistatic: the effect of one gene hides the effect of the other gene

Recessive epistasis

Addition of A or B sugars

H allele is epistatic to the I gene

Fig. 3.14

Dominant epistasis

A produce particular color, but B dominant allele epistatic to A

Table 3.2

Four classes of genotypes produce a variety of phenotypic ratios

Imcomplete dominance in interaction of two genes

Fig. 3.15

Genetic Heterogeneity

Heterogeneous trait

A mutation at any one of a number of genes can give rise to the same phenotype

Fig. 3.18

Pedigree

analyses



b. Multiple alleles



a. Two genes can interact to determine one trait

b. Heterogeneous trait

c. The same genotype does not always produce the same phenotype

Extension to Mendel

The same genotype does not always produce the

same phenotype

1. Modifier genes

2. Environment

Penetrance: occurrence in population

Expressivity: seriousness in the individuals

Modifier genes

Major genes have a large influence, while modifier genes

have a more subtle, secondary effect.

Modifier genes alter the phenotypes produced by the allele

of other genes.

Example: tail length of mouse

T allele: 10%, 50%, 75% of the normal tail-length

Fig. 3.19

Permissive temp.

Restrictive temp.

The Environment can affect the phenotypic expression

Even continuous variation can be explained by extensionsto Mendelian analysis

The more genes or alleles, the more possible phenotypicclasses, and the greater the similarity to continuous variation

Fig. 3a.p64

TABLES

Fig. 3.1

Variations on complete dominance do not negate

Mendel’s law of Segregation

Fig. 3.8 Plant incompatibility system promote outbreeding and allele proliferation

Fig. 3.9

Some alleles may

cause lethality

Fig. 3.17

Breeding studies help decide how a trait is inherited

CO 03 Extension to Mendel: complexities in relating genotype to phenotype.

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