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 C_P_ : 3 C_pp :3 ccP_ : 1 ccpp purplewhite

Epistatic Gene Interaction Complementary gene action Enzyme C and enzyme P cooperate to make a product, therefore they complement one another Enzyme CEnzyme 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 CEnzyme 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 2 nd 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 CrossOutcome P cross: Cream-eyed male X wild-type female F 1 : all red eyes F 1 cross: F 1 brother X F 1 sister F 2 : 104 females with red eyes 47 males with red eyes 44 males with eosin eyes 14 males with cream eyes F 2 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 CrossOutcome P cross: Cream-eyed male X wild-type female F 1 : all red eyes F 1 cross: F 1 brother X F 1 sister F 2 : 104 females with red eyes 47 males with red eyes 44 males with eosin eyes 14 males with cream eyes F 2 generation contains – eye: 44 w e eye: 14 c a eye a 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 w e allele, while the wt cream allele does not C = Normal allele Does not modify the eosin phenotype c a = Cream allele Modifies the eosin color to cream, does not effect wt or white allele of white gene.

Male gametes CYCY CCX w+ X w+ CCX w+ Yc a c a X w+ X w+ Cc a X w+ YCX w+ c a X w+ caYcaY CX w-e c a X w+ c a X w-e CCX w+ X w-e CCX w-e YCc a X w+ X w-e Cc a X w-e Y Cc a X w+ X w+ Cc a X w+ Yc a c a X w+ X w+ c a c a X w+ Y Cc a X w+ X w-e Cc a X w-e Y c a c a X w+ X w-e c a c a X w-e Y Female gametes Putative genotypes in a cross P w + / w + ; C/C x w e /Y; c a /c a F 1 w + / w e ; C/c a & w + /Y; C/c a F 2 ¾ C/_ x ¾ w + /_ ¼ w e /Y ¼ c a /c a x ¾ w + /_ ¼ w e /Y 9/16 C/_ ; + 3/16 c a /c a ; + 3/16 C/_ ; w e 1/16 c a /c a ; w e Modeling the Data red eosin cream 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

F 2 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 TTVVTTVvTtVVTtVv TTVvTTvvTtVvTtvv TtVVTtVvttVVttVv TtVvTtvvttVvttvv (b) The crosses of Shull TTVV Triangular ttvv Ovate TtVv All triangular F 1 (TtVv) x F 1 (TtVv) x F 1 generation 15:1 ratio results

Bombay Phenotype

Categories of Inheritance Paterns

Complementary action Duplicate action Epistasis of aa over B-Epistasis of A- over bb Generation of Epistatic Ratios