Question

a couple has just learned that their daughter has hemophilia. They already have a son who does not have the disease. What are the genotypes of the couple? Explain your reasoning.

Answer 1

Answer: The genotypes of the couple are- XᵇX and XᵇY, where Xᵇ denotes the X chromosome having gene for hemophilia.

Hemophilia is a X linked recessive disorder, which means that the gene for hemophilia is carried by X chromosome. A female individual will be affected by the disorder if she carries two copies of the gene causing hemophilia. A male ( XY) will be hemophilic when his X chromosome bears the gene for hemophilia.

As per the information given in the question, son is not having the disease, which means that he has inherited normal X chromosome from his mother. On the other hand, daughter is hemophilic, which indicates that she has inherited both the X chromosomes with hemophilic gene that is XᵇXᵇ ( one X from father and other from mother).

This condition is only possible when the male parent is affected and female parent is carrier for the disease.

Thus, the genotypes of the couple are- XᵇX and XᵇY, where Xᵇ denotes the X chromosome having gene for hemophilia.