The agouti locus in mice has three alleles a is recessive an

The agouti locus in mice has three alleles: a is recessive and confers a black coat, A is dominant to a and confers an agouti coat, and A^Y is dominant to both and confers a yellow coat. A^Y is also a recessive lethal. In addition, the independently assorting c allele is recessive (to C) and prevents the mice from making any fur pigments at all, resulting in albinos. Two mice are crossed, and produce offspring in a ratio of 2 yellow : 1 albino : 1 agouti. What are the genotypes and phenotypes of the parent mice?  

Solution

In this cross, we need to consider two main factors to derive the parent phenotype and genotype

1) A^Y in homozygous state is lethal

2) cc would result in albino irrespective of A or A^Y

By observing the phenotypes of the cross we see the following

a) We find both coloured and albino mice. that means to say that C is in heterozygous condition (C,c)

b) We observe both yellow and agouti phenotype and hence A loci is also heterozygous (A, A^Y)

With this information, if we make a punnet square for cross AA^YCc:

You can see after excluding the lethal types we get 6:3:3 which is same as 2:1:1 for yellow : albino : agouti.