In humans, cystic fibrosis is an inherited disease due to an autosomal recessive gene located on chromosome #7. In the most common defective allele, three base pairs are deleted and a single phenylalanine is missing. Affected individuals carry two of the recessive alleles for the disease (genotype ff) and as a result form extremely thick mucus in their respiratory systems and elsewhere in the body. Their lungs are susceptible to frequent infections and the disease is progressive and eventually fatal. Usually the victims die in their teens or early twenties, and so do not reproduce. Among whites one person in 20 is a carrier, who is heterozygous (Ff) for cystic fibrosis.
Severe natural selection has operated on the gene pool for cystic fibrosis (f) and normal Alleles (F) over the centuries. The affected individuals do not reproduce and do not pass on their genes. New cases arise only when both parents are heterozygous or through new mutations in a "normal" parent.
In the following exercise, you will compare the effects of natural selection alone and natural selection plus negative eugenics on the frequency of the f allele in a model system. Among the children of marriages between heterozygous carriers of the f allele and genetically normal individuals, the frequency of the f allele should be 25%, the same as in the parents. We will use that frequency at the start of both this and the following experiments.