Chapter 11, Problem 34P

Approximately 3% of the population carries a mutant allele at the CFTR gene responsible for the recessive disease cystic fibrosis. A genetic counselor is examining a family in which both parents are known to be carriers for a CFTR mutation. Their first child was born with the disease, and the parents have come to the counselor to assess whether the new fetus inside the mother’s womb is also diseased, is a carrier, or is homozygous wild type at the CF locus. DNA samples from each family member and the fetus are tested by PCR and gel electrophoresis for an SSR marker within one of the CFTR gene’s introns. The following results are obtained:

 

a.	What is the probability that the child who will develop from this fetus will exhibit the disease?
b.	When this child grows up, what is the probability that any one of her own children will be afflicted with the disease?
c.	The cystic fibrosis gene displays extensive allelic heterogeneity: More than 1500 different mutations of the CFTR gene have been shown to be associated with cystic fibrosis worldwide. With this fact in mind, why might human geneticists choose to test the fetus in the indirect manner described in this problem rather than focusing directly on the mutations that actually caused the disease in the first child?