Question

What is the probability that an offspring will inherit cystic fibrosis, an autosomal recessive disorder, if both parents are carriers?

Answer 1

There is a 1/4 chance that the offspring will have the disease and a 1/2 chance the offspring will be a carrier but not express the disease.

Explanation:

There is a 1/4 chance that the offspring will have the disease and a 1/2 chance the offspring will be a carrier but not express the disease. This leaves a 1/4 chance that the offspring will neither have the disease nor be a carrier.

Remember, recessive genes require two copies to be expressed. This is in contrast to dominant genes, which are expressed even when there is only one copy present.

In the image below, the third generation includes two individuals who are carriers, one who is not a carrier, and one who has CF. The parents of the third generation are both carriers, meaning for each child they have, each parent has a 50% chance of passing on the CF mutation. Thus, the likelihood of a child born from these parents having CF is 1/4 or (1/2)*(1/2).

If one parent was a carrier and the other parent was not a carrier, the offspring would have a 1/2 chance of being a carrier and a 0% chance of having CF (see the first generation on the right side of the image).