Heterozygous Cp cp chickens express a condition called creeper, in which the leg and wing bones are shorter than normal (cp cp). The dominant Cp allele is lethal when homozygous. Two alleles of an independently segregating gene determine white (W-) versus yellow (ww) skin color. From matings between chickens heterozygous for both of these genes, what phenotypic classes will be represented among the viable progeny and what are their expected relative frequencies

Dihybrid cross:  Cp cp W w  x  Cp cp W w

Gametes= CpW; Cpw; cpW; cpw

Cp Cp: lethal phenotype  

Genotypes:

Cp cp W w >> creeper and white phenotype

Cp cp w w  >> creeper and yellow phenotype  

Cp cp W W >> creeper and white phenotype

cp cp W W >> normal (non-creeper) and white phenotype

cp cp W w >> normal (non-creeper) and white phenotype

cp cp w w >> normal (non-creeper) and yellow phenotype

Since it is a dihybrid cross that involves a lethal allele in homozygous condition (Cp), it is expected a phenotype ratio of 6:3:2:1, it is 6 (Cpcp W_ creeper white) : 3 (cpcp W_ normal white ) : 2 (Cpcp W_ creeper yellow) : 1 (cpcp ww normal yellow). This ratio is expected because it is a distortion from normal segregation proportions (i.e., instead of the 9:3:3:1 phenotypic ratio associated with dihybrid crosses).