Question

The offspring of first cousins have an increased chance of combining the same, sometimes deleterious, allele (higher probability of identity by descent). Cystic fibrosis is a frequently fatal disease causes by homozygosity for a recessive allele of the CFTR gene. This allele is present in the US population ~0.00006. Based on this value, what is the expected frequency of this disease in a large, randomly mating population? What is the expected frequency among children of first cousin marriages? With this information, how much more likely is cystic fibrosis to occur in children from first cousins versus unrelated individuals?

Answer 1

In first cousin marriages, they have same grandparents, hence chances of recombination is less as compared to random mating. If one of the grandparents is carrier for trait (Cc genotype), there are 50% chances that the child of grandparent will have Cc as genotype (frequency of c =0.5). Assuming random mating, there will be 25% chances of Cc as genotype in cousins (frequency of c =0.25). If cousins marry with each other, their children will have genetic ratio of 1(CC, normal) :2 (Cc, carrier):1 (cc,affected),  

frequency of c will be 0.0625, resulting in 25% normal :50% carriers: 25% affected individuals

This frequency is vary high as compared to the frequencies of randomly mated population