Answer to Question #83160 in Genetics for Angelica Flores

Question

A woman with hemophilia (a recessive, sex-linked condition) has a son with hemophilia. The son marries a woman whose father has hemophilia, but she does not.
What is the probability that a daughter of this mating will be a hemophiliac?
A son?
If the couple has 3 sons what is the probability that all of them will NOT be hemophiliacs?

Answer 1

Let X* denote chromosome with allele for hemophilia and X denote the chromosome with normal allele.

The man's genotype is X*Y and his wife's genotype is X*X.

Their daughter could have either X*X or X*X* genotype. Thus, the probability for the daughter to be hemophiliac is 1/2.

The son could have genotype X*Y or XY. Thus, the probability to be hemophiliac is 1/2 as well.

Birth of each son is the independent event. Thus, the probability is 1/2 * 1/2 *1/2 =1/8.