A woman with normal blood clotting (heterozygote) and a man with hemophilia are married.

A woman with normal blood clotting (heterozygote) and a man with hemophilia are married. What is the probability of birth in this family of children with hemophilia, if it is known that hemophilia is a recessive trait linked to the X chromosome?

Let’s designate the chromosome with the pathological gene that causes hemophilia as Xg. The chromosome with the gene that determines the development of the normal blood coagulation system will be designated as XG.

A man with this coagulation pathology will be recorded as Xg Y, he will produce two types of sperm – Xg and Y.

A woman heterozygous for a pathological gene will be healthy and have a genotype

XG Xg. It will produce two types of eggs, XG and Xg.

All possible offspring of this married couple will be represented by the following options:

girls with normal blood clotting, carriers of the pathological gene (XG Xg) – 25%;

boys with hemophilia (Xg Y) – 25%;

girls with hemophilia (Xg Xg) – 25%;

boys with normal blood coagulation system (XG Y) – 25%.

Answer: The probability of having a child with hemophilia is 50%.