Answer
2:1 yellow:wild type
Work Step by Step
Since the yellow allele is lethal in the homozygous condition, all homozygous yellow mice will die during early development, and none will appear in the generation which is born. The yellow x yellow cross is a cross between two heterozygous mice, which should result in a 1:2:1 distribution of genotypes, but with the homozygous yellow individuals dying, only the 2:1 part of the population is left.
