Molecular Genetics: Problem 1

A man with hemophilia (a recessive , sex-linked condition has a daughter of normal phenotype. She marries a man who is normal for the trait. What is the probability that a daughter of this mating will be a hemophiliac? A son? If the couple has four sons, what is the probability that all four will be born with hemophilia?

Genotypes:

A man with hemophilia is X^hY where h = hemophilia gene and H = the normal gene.
Any daughter with normal phenotype whose father has hemophilia will be a carrier.

Her genotype must be:

X^hX^H and not X^HX^H

We can use a Punnett square to show the probability of a daughter or son having hemophilia.

daughter x normal man

X^hX^H x X^HY

A. If the daughter marries a normal male the probability of a daughter having hemophilia is zero.
B. About 50% of male children would have hemophilia (Boxes 2 and 4 above)

C. The probability that all 4 sons have inherited hemophilia would be: 1/2 x 1/2 x 1/2 x 1/2 or 1/16.