Question

29. A genetic testing company offers expanded carrier screening. Amongst the conditions they test for, they provide the following information regarding the carrier frequency and testing sensitivity for cystic fibrosis (CF) in different populations.

Ethnicity	Carrier frequency	Testing sensitivity
African American	1 in 61	97%
Ashkenazi Jewish	1 in 29	93%
Asian	1 in 88	98%
Caucasian	1 in 28	92%

A couple consisting of a Caucasian man and an Asian woman are interested in doing this expanded carrier screening test.

1. Assuming there are no known people affected with CF in either of their families, what are the baseline (i.e. before testing) chances of this couple having a child affected with CF?

2. The woman does testing first, and she is found to be a CF carrier. Taking this finding into account, what are the updated chances of this couple having a child affected with CF?

3. The man does testing, and results show he has not been detected as a carrier. Taking both the man’s and the woman’s testing results into account, what are their updated chances of having a child with CF?

Answer 1

To have cystic fibrosis, a child must inherit one copy of the cystic fibrosis transmembrane conductance regulator (CFTR) gene mutation from each parent. People who have only one copy of a CFTR gene mutation do not have CF. They are called "CF carriers."

Each time two CF carriers have a child, the chances are:

• 25 percent (1 in 4) the child will have CF
• 50 percent (1 in 2) the child will be a carrier but will not have CF
• 25 percent (1 in 4) the child will not be a carrier of the gene and will not have CF

People with CF can also pass copies of their CF gene mutations to their children. If someone with CF has a child with a CF carrier, the chances are:

• 50 percent (1 in 2) the child will be a carrier but will not have CF
• 50 percent (1 in 2) the child will have CF