A woman whose brother had cystic fibrosis marries a man who had a child with cystic fibrosis from a previous marriage. They plan to have 3 children. What is the that only one of the 3 will have cystic fibrosis?
First thing that we must note is that cystic fibrosis is caused by double dose of a recessive allele in genotype. The gene is located on chromosome 7 and it follows mendelian pattern of inheritance.
A woman whose brother had cystic fibrosis could be a carrier for the wrong allele. She does not suffere from the disease despite being a carrier of the recessive allele on one chromosome, due to presence of the corresponding normal allele on the homologous pair.
If the man already had a child with cystic fibrosis then he is also a carrier of the disease. A child must inherit two recessive alleles to become affected by the disease: each allele comes from one carrier parent. Thus, there is possibility of inheriting the disease in one of every four children. We may otherwise say that in case of every pregnancy, there is about 25% chance of having an affected child.