Question

Flies homozygous or hemizygous for the w¹ allele of the white gene (X^w1/X^w1 or X^w1/Y) have white eyes, whereas flies homozygous or hemizygous for the w^a allele of the white gene (X^wa/X^waor X^wa/Y) have orange eyes.

      What is the dominance relationship between the w¹ and the w^a alleles, knowing that in the offspring of a cross between white-apricot females and white males the heterozygous females of the F₁ generation (X^wa/ X^w1) have lighter orange eyes than homozygous X^wa/X^wa parental females? How can you explain this relationship?

      How can you explain that the hemizygous X^wa/Y males of the F₁ generation have darker orange eyes than the heterozygous females of the F₁ generation, although both carry one X^wa allele?

Answer 1

      What is the dominance relationship between the w1 and the wa alleles, knowing that in the offspring of a cross between white-apricot females and white males the heterozygous females of the F1 generation (Xwa/ Xw1) have lighter orange eyes than homozygous Xwa/Xwa parental females? How can you explain this relationship?

In this case, we are observing a trait that follows a X-linked inheritance patter but at the same time, the trait follows an incomplete dominance patter. When incomplete dominance occurs, the dominant and recessive allele will form a intermediate phenotype. In this case, orange and white will form a lighter orange.

      How can you explain that the hemizygous Xwa/Y males of the F1 generation have darker orange eyes than the heterozygous females of the F1 generation, although both carry one Xwa allele?

As we mentioned, if the trait follows an incomplete dominance, individuals that only posses one type of allele will fully express the phenotype of that allele. For this reason, males that have only one allele for orange will show the true darker orange color.