A man with normal blood clotting is excited by the news that his wife’s sister has given birth to a hemophilic

A man with normal blood clotting is excited by the news that his wife’s sister has given birth to a hemophilic boy (the man is thinking about the health of his future children). How could he be reassured by the news that hemophilia had never been observed among his wife’s maternal relatives?

Chromosome, in which the recessive gene that causes the development of hemophilia is inherited – X h. A chromosome with a gene that determines the development of a normal blood coagulation system – X H.

A man with a normal blood coagulation system will have the X H Y genotype

and produce sperm cells X H and Y.

The woman, among whose maternal relatives has never been observed hemophilia, is healthy, but her sister gave birth to a child with hemophilia. This means that on the paternal side, a woman could inherit a recessive pathological gene and be its carrier. A woman’s genotype can be either homo- or heterozygous.

Let’s consider both options.

Suppose, on the father’s side, a woman also could not inherit the hemophilia gene, then she is homozygous and has the X H X H genotype. Her body only produces eggs containing the gene for normal blood clotting X H. In a marriage with a healthy man, all descendants will be healthy: X H Y – 50%, X H X H – 50%.

If a healthy woman still receives a pathological gene on the paternal side, then she will have a heterozygous genotype X H X h and produce two types of eggs – X H and X h. In a marriage with a healthy man, the following offspring options are possible:

healthy girls (X H X H) – 25%;

healthy heterozygous girls – carriers of the pathological gene (X H X h) – 25%;

healthy boys (X H Y) – 25%;

boys with hemophilia (X h Y) – 25%.

The fact that maternal hemophilia was not observed in the wife’s family does not exclude the birth of a child with hemophilia in the event that his wife is heterozygous.