Question

You are working as a genetic counsellor.

A newly married couple comes to you for advice. Neither the man nor the woman has cystic fibrosis, but both have full siblings with this genetic disease. Cystic fibrosis is an autosomal recessive disease caused by a single locus and is typically diagnosed by the time a child is 2 years old. The young couple are planning to start a family and are worried about the risk that they might conceive a child with cystic fibrosis. Based on the information provided here only, what is the probability that a child of this couple would have cystic fibrosis?

Answer 1

Both man and woman do not have cystic fibrosis. However, both of their siblings have it. Since cystic fibrosis is an autosomal recessive disorder, it will be exhibited only when an individual has two recessive alleles. Let the normal dominant allele be C and the affected recessive allele be c. Hence the siblings are cc. Therefore, the siblings have received one recessive allele from both their parents. Hence, the man and woman will be Cc as they will have a single allele being contributed from one of their parents.

Crossing these parents, we get:

	C	c
C	CC	Cc
c	Cc	cc

cc is present in one out of the four offsprings. Hence, the probability that this couple will have a child with cystic fibrosis is 1/4 = 25%.