Assume that a dihybrid F2 ratio resulting from epistasis was

Assume that a dihybrid F2 ratio, resulting from epistasis, was 9:6:1. If a double heterozygote was crossed with the fully recessive type, what phenotypic ratio is expected among the offspring?

a. 1:1 b. 9:7 c. 1:2:1 d. 2:1 e. 3:1

Solution

Answer: c. 1:2:1

Duplicate genes with cumulative effect (9:6:1):

Certain phenotypic traits depend on the dominant alleles of two gene loci. When two tdominant is present it will show its phenotype. The ratio will be 9: 6: 1.

It means that during dihybrid cross, either of dominant genes or both dominant genes give similar phenotype.

A_bb or aaB_ all these genotypes give the same phenotype. So the Dihybrid cross ratio will be changed from 9:3:3:1 to 9:6:1.

If a double heterozygote was crossed with the fully recessive type,

AaBb x aabb----------Parents

ab

AB

AaBb

Ab

Aabb

aB

aaBb

ab

aabb

1:2:1