Question

Hemophilia is caused by several genetic factors; one, a recessive allele of an X- linked gene, is the subject of this problem. Assume that a man with hemophilia marries a normal woman whose father had hemophilia. What is the probability that their first son will have hemophilia?

1. 1/16

2. 1/8

3. 1/4

4. 1/2

5. 3/4

Answer 1

The correct answer is 4. 1/2

Explanation: The genotype of a man with hemophilia: X^hY where h is a recessive allele of the hemophilia gene. [ Hemophilia is an X-linked recessive disorder]

A normal woman whose father had hemophilia is obviously the carrier for hemophilia because she will get one X chromosome from her father. So, her genotype: X^HX^h where H is the dominant allele of the hemophilia gene.

Now, the man mate with the woman.

So, if we use the punnet square,

	Xh	Y
XH	XHXh	XHY
Xh	XhXh	XhY

X^HX^h : Carrier daughter.

X^HY: Normal son.

X^hX^h: Hemophilic daughter.

X^hY: Hemophilic son.

Therefore, the probability that their first son will have hemophilia= 1/2 (Here we do not have to add the probability for sex to the solution because we are only considering males here).