In Mice,a Cross Between A^YA⁺ Heterozygotes Produces Two Kinds of Progeny,yellow

In mice,a cross between A^YA⁺ heterozygotes produces two kinds of progeny,yellow (A^YA⁺)and gray-brown (A⁺A⁺),in a ratio of 2:1.Why is there an altered ratio of individuals produced?
A) A^YA^Yindividuals die during embryonic development due to the presence of a lethal mutation
B) A^YA⁺individuals are physiologically more able to adapt to the birth process
C) A⁺A⁺ individuals are the mutant phenotype and occur more frequently in nature
D) A^YA⁺ individuals are the mutant phenotype and occur more frequently in nature
E) None of these explains why there would be an altered ratio in this case