Non-functional alleles are said to be:

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

The test to determine whether similar phenotypic mutations are alleles of the same gene based on the phenotypic effect of combining the mutations in the same individual is known as the: