# A 28-year-old woman who is eight weeks pregnant attends a genetic counselling service with her husband

#1

A 28-year-old woman who is eight weeks pregnant attends a genetic counselling service with her husband, who is a lawyer. His brother is heterozygous for the Δ508 mutation for cystic fibrosis. The husband agrees to be screened and is also found to be heterozygous for the mutation. The pregnant woman in question declines the screening test. Assuming that the Δ508 mutation has a prevalence of 1:20 in the British population, what is the chance of her having a child with the condition?

a. 1 in 20.
b. 1 in 80.
c. 1 in 400.
d. 1 in 160.
e. 1 in 4.

Explanation:

The gene frequency of the mutation in question is 1/20, hence the chances of the pregnant woman carrying the mutation is 1 in 20. Her husband has been screened and is a definite carrier. Therefore, the chances of them both having a heterozygous genetic mutation is
1/20 1/1 = 1/20.
If both partners are heterozygous for an autosomal recessive disorder such as cystic fibrosis, the chances of a child being affected is 1 in 4. Therefore the overall chances of the couple having a child with cystic fibrosis is 1/20 1/4 = 1/80.