Practice question

X-linked genes reside on X chromosome where dosage differs between XX females and XY males. In Drosophila, sons receive X from mother and Y from father, daughters receive X from both. Consequently swapping mutant and wild type between sexes changes which sex inherits mutant X, altering progeny phenotypes. For X-linked recessive white eye, one reciprocal direction yields white-eyed sons and red-eyed daughters, opposite reciprocal yields different distribution. Such non-equivalence between reciprocals contrasts with autosomal crosses showing identical outcomes and signals X-chromosome location and hemizygous male expression.