Question

Hemophilia is a sex-linked recessive disorder. If an unaffected woman mates with an unaffected man, and they have 2 children with hemophilia, and 2 children who are unaffected

What are the chances that the couple will have a boy who has hemophilia?

Answer 1

As both the parents are unaffected but they have affected children, the genotype of mother will be -X^aX , she will carry a gene for hemophilia in one X chromosome. But as she has another X chromosome normal and hemophilia is sex linked recessive disorder, she will be not affected by disorder. On the other hand the father will be XY.

X^aX X XY

	Xa	X
X	XaX unaffected, female (carrier)	XX unaffected female
Y	XaY affeced , male	XY normal, male

there is a 25% chance that the couple will have one son who will have hemophilia.