One form of hemophilia is inherited as a recessive trait linked to the X chromosome. A man with hemophilia marries a normal woman (whose father and mother are healthy). Will children suffer from this disease?
Let us designate the gene that determines the development of hemophilia g in a person, then the chromosome in which this gene is inherited will be designated X g. The chromosome with the gene that determines the normal blood coagulation system will be designated XG.
A man with hemophilia has the X g Y genotype and produces two types of sperm, X g and Y.
The woman’s healthy father had the X G Y genotype. To his daughter, he could only pass on X G, a chromosome with the normal blood clotting gene.
The woman’s mother was healthy, since she inherited the XG chromosome from her healthy father, however, she could be both homo- and heterozygous for hemophilia. Let’s consider both options.
Let’s say the woman’s mother was homozygous – X G X G. Then the woman is also homozygous – X G X G. She produces eggs X G.
The offspring from a marriage with a healthy male will be represented by healthy boys (XGY) and healthy homozygous girls (X G X G) with a probability of occurrence corresponding to a ratio of 1: 1.
Let’s say a woman’s mother was heterozygous and gave her a chromosome with a pathological gene X g. Then the woman is heterozygous – X G X g. It produces two types of eggs – X G and X g. The offspring from a marriage with a healthy man will be represented by the following possible options:
healthy homozygous girls (X G X G) – 25%;
healthy heterozygous girls (X G X g) – 25%;
healthy boys (X G Y) – 25%;
boys with hemophilia (X g Y) – 25%.
Answer: if a woman is homozygous, then the probability of having a sick child is 0%. If the woman is heterozygous – 25%.