Question

Haemophilia is an X-linked disorder that occurs at a frequency of approximately 1 in 10000.

A normal male and a female who is heterozygous for the haemophilia mutation are expecting a child. What is the probability of this couple having a child with haemophilia? (Assume that all carrier females are asymptomatic.)

Answer 1

The probability is 25%.

There are 4 different genotypes possible in children: homozygous dominant girl, heterozygous girl, a boy with dominant allele that protects against the disease, and a boy with recessive allele. Only the last one will suffer from the disease.