Gene interactions occur when two or more different genes influence the outcome of a single trait

Most morphological traits (height, weight, color) are affected by multiple genes

Epistasis describes situation between various alleles of two genes

Quantitative loci is a term to describe those loci controlling quantitatively measurable traits

Pleiotropy describes situations where one gene affects multiple traits

Epistatic Gene Interactions

examine cases involving 2 loci (genes) that each have 2 alleles

Crosses performed can be illustrated in general by AaBb X AaBb Where A is dominant to a and B is dominant to b

If these two genes govern two different traits A 9:3:3:1 ratio is predicted among the offspring simple Mendelian dihybrid inheritance pattern

If these two genes do affect the same trait the 9:3:3:1 ratio may be altered 9:3:4, or 9:7, or 9:6:1, or 8:6:2 or 12:3:1, or 13:3, or 15:1 epistatic ratios

Epistatic Gene Interactions

A Cross Producing a 9:7 ratio

Figure 4.18

9 C_P_ : 3 C_pp :3 ccP_ : 1 ccpp

purple white

Epistatic Gene Interaction

Complementary gene action Enzyme C and enzyme P cooperate to

make a product, therefore they complement one another

Enzyme C Enzyme P

Purple pigment

Colorless intermediate

Colorless precursor

Epistasis describes the situation in which a gene masks the phenotypic effects of another gene

Epistatic interactions arise because the two genes encode proteins that participate in sequence in a biochemical pathway

If either loci is homozygous for a null mutation, none of that enzyme will be made and the pathway is blocked

Colorless precursor

Colorless intermediate

Purple pigment

Enzyme C Enzyme P

Epistatic Gene Interaction

genotype cc

genotype pp

Colorless precursor

Colorless intermediate

Purple pigment

Enzyme C Enzyme P

Inheritance of the Cream-Eye allele in Drosophila a rare fly with cream-colored eyes identified in a

true-breeding culture of flies with eosin eyes possible explanations

1. Mutation of the eosin allele into a cream allele 2. Mutation of a 2nd gene that modifies expression of the

eosin allele

Epistasis of Involving Sex-linked Genes

The Hypothesis

Cream-colored eyes in fruit flies are due to the effect of a second gene that modifies the expression of the eosin allele

Figure 4.19

Testing the Hypothesis

cream allele is recessive to +

Interpreting the Data

Cross OutcomeP cross:

Cream-eyed male X

wild-type female

F1: all red eyes

F1 cross:

F1 brother X F1 sister

F2: 104 females with red eyes

47 males with red eyes

44 males with eosin eyes

14 males with cream eyes F2 generation contains males with eosin eyes

This indicates that the cream allele is not in the same gene as the eosin allele

Interpreting the Data

Cross OutcomeP cross:

Cream-eyed male X

wild-type female

F1: all red eyes

F1 cross:

F1 brother X F1 sister

F2: 104 females with red eyes

47 males with red eyes

44 males with eosin eyes

14 males with cream eyes F2 generation contains –

151 + eye: 44 we eye: 14 ca eyea 12 : 3 : 1 ratio

Modeling the Data

Cream phenotype is recessive therefore the cream allele is recessive allele (either sex-linked or autosomal)

The mutated allele of the cream gene modifies the we allele, while the wt cream allele does not C = Normal allele

Does not modify the eosin phenotype ca = Cream allele

Modifies the eosin color to cream, does not effect wt or white allele of white gene.

Male gametes

CY

CCXw+Xw+ CCXw+Y cacaXw+Xw+ CcaXw+YCXw+

CXw+ caXw+ caY

CXw-e

caXw+

caXw-e

CCXw+Xw-e CCXw-eY CcaXw+Xw-e CcaXw-eY

CcaXw+Xw+ CcaXw+Y cacaXw+Xw+ cacaXw+Y

CcaXw+Xw-e CcaXw-eY cacaXw+Xw-e cacaXw-eY

Fem

ale

gam

etes

Putative genotypes in a crossP w+/ w+; C/C x we/Y; ca/ca

F1 w+/ we; C/ca & w+/Y; C/ca

F2 ¾ C/_ x ¾ w+/_ ¼ we/Y

¼ ca/ca x ¾ w+/_ ¼ we/Y

9/16 C/_ ; +3/16 ca/ca; +3/16 C/_ ; we

1/16 ca/ca; we

Modeling the Data

red

eosincream

12:3:1

Inheritance of comb morphology in chicken First example of gene interaction William Bateson and Reginald Punnett in 1906 Four different comb morphologies

A Cross Involving a Two-Gene Interaction Can Still Produce a 9:3:3:1 ratio

Figure 4.17b

The crosses of Bateson and Punnett

F2 generation consisted of chickens with four types of combs 9 walnut : 3 rose : 3 pea : 1 single

Bateson and Punnett reasoned that comb morphology is determined by two different genes R (rose comb) is dominant to r P (pea comb) is dominant to p R and P are codominant (walnut comb) rrpp produces single comb

Gene Interaction

Duplicate gene action Enzyme 1 and enzyme 2

are redundant They both make product

C, therefore they duplicate each other

Duplicate Gene Action Epistasis

TV

TV

Tv

Tv

tV

tV

tv

tv

TTVV TTVv TtVV TtVv

TTVv TTvv TtVv Ttvv

TtVV TtVv ttVV ttVv

TtVv Ttvv ttVv ttvv

(b) The crosses of Shull

TTVVTriangular

ttvvOvate

TtVvAll triangular

F1 (TtVv) x F1 (TtVv)

x

F1 generation

15:1 ratio results

Bombay Phenotype

Bombay Phenotype

Bombay Phenotype

Categories of Inheritance Paterns

Com

plem

enta

ry a

ctio

n

Dup

lica

te a

ctio

n

Epi

stas

is o

f aa

ove

r B

-

Epi

stas

is o

f A

- ov

er b

b

Generation of Epistatic Ratios