Cystic fibrosis results from mutations in a single gene that lies on Chromosome 7. Only homozygous mutant (ff) individuals are sick; homozygous wild-type (FF) and heterozygous (Ff) individuals are healthy
A healthy married couple has one child with cystic fibrosis and the wife is pregnant with a second child.
A. What is the genotype of the mother? The father?
B. What is the chance that the second child will have cystic fibrosis?
A. The genotypes of the mother and father are the same: Ff. The only way that a child can have the disease is if both parents are carriers of the mutant cystic fibrosis gene.
B. The chance that the second child will have cystic fibrosis is one-quarter. The chance that the mother will transmit her mutant f allele to the offspring is one-half, multiplied by an equal chance that the father will transmit his mutant f allele to the offspring: 1/2 × 1/2 = 1/4. (Having one child with cystic fibrosis does not change the probability of having another child with the disease.)
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