Hemophilia is a sex-linked recessive trait (Y). Can a normal man and woman have children with hemophilia?

Probably, when writing down the condition of the problem, an inaccuracy was made: it is well known that the gene that determines the development of hemophilia in humans is inherited in the X chromosome. Let’s designate such a chromosome as Xg and solve the problem taking into account the amendment.

The chromosome with the gene for the normal coagulation system will be designated respectively – XG.

A healthy man, being a representative of the heterogametic sex, will have the XGY genotype and produce two types of sperm – XG and Y. Thus, he will transfer a chromosome with a healthy gene to girls, and a Y chromosome that is neutral in relation to hemophilia, which will determine their development according to the male type …

Since all daughters will inherit a chromosome with a dominant gene for the normal blood coagulation system from a healthy father, they will all be healthy.

Boys with hemophilia have the Xg Y genotype, inheriting a chromosome with the abnormal Xg gene from their mother. In order to be a healthy mother of a boy with hemophilia, a woman must be heterozygous, that is, be a hidden carrier of a pathological gene. Her genotype should look like this – XG Xg. A woman with this genotype is capable of producing eggs with both the normal blood coagulation gene and the hemophilia gene.

All possible options for the offspring of a heterozygous woman and a healthy man are represented by the following options:

homozygous healthy girls (XG XG) – 25%;

healthy boys (XG Y) – 25%;

heterozygous healthy girls (XG Xg) – 25%;

boys with hemophilia (Xg Y) – 25%.

Answer: a healthy man and woman can have a child with hemophilia with a probability of 25%, provided that the mother’s genotype is heterozygous (hidden carriage of the pathological hemophilia gene).