One true-breeding line of mice is obese and dark, and the other true-breeding line is lean and light.Dark is dominant to light, but obese and lean exhibit incomplete dominance.The two genes involved are on separate chromosomes.What proportion of offspring from a dihybrid cross should be dark and be intermediate between obese and lean?
A) 1/16
B) 3/16
C) 1/4
D) 3/8
E) 9/16

Question

Quizplus · Accepted Answer

The dihybrid cross can be written as:

DDOO x ddoo

where D represents the dominant allele for dark color, d represents the recessive allele for light color, O represents the dominant allele for obese, and o represents the recessive allele for lean.

The gametes produced by the first parent are all DO, and the gametes produced by the second parent are all do. The possible genotypes and phenotypes of the offspring are:

DdOo (dark and intermediate) 	9/16
Ddoo (dark and lean) 		3/16
ddOo (light and intermediate) 	3/16
ddoo (light and lean) 		1/16

Therefore, the proportion of offspring that are dark and intermediate between obese and lean is 9/16, which corresponds to choice D.

One True-Breeding Line of Mice Is Obese and Dark, and the Other