What genotypes do the parents have if their offspring include two daughters with genotype XaXA, a son with genotype XaYa, and another son with genotype XaYA?

To determine the correct genotypes of the parents, it's essential to analyze the offspring's genotypes. The daughters exhibit the genotype XaXA, indicating that each daughter has inherited one X chromosome carrying the Xa allele from each parent. The presence of two daughters with the same genotype reinforces that both parents must contribute the Xa allele.

The sons have genotypes XaYa and XaYA. The son with XaYa has inherited the Xa allele from one parent and the Y chromosome from the other parent, which indicates that the father must possess a Y chromosome since males inherit their Y chromosome from their father. The other son with genotype XaYA similarly inherits the Xa from one parent and the YA from the other parent, suggesting that he also received the Xa allele from the mother while inheriting the Y chromosome from the father.

Based on the information presented, it becomes apparent that the mothers must be providing the Xa allele to both daughters and both sons. Since the daughters' genotype is XaXA and the sons have the Xa allele as well, this indicates that the father must contribute the Y chromosome but not a dominant X allele, ruling out any dominant X allele contribution from him.

Considering the offspring's patterns, the only combination that aligns with all these requirements is that both parents contribute