A rare recessive allele inherited in a Mendelian manner causes the disease cystic fibrosis. A phenotypically normal man whose father had cystic fibrosis marries a phenotypically normal woman from outside the family, and the couple consider having children. a.) draw the pedigree as far as described? b.) If the frequency in the population of heterozygotes for cystic fibrosis is 1 in 50, what is the chance the couples first child will have cystic fibrosis? c.) If the first child does have cystic fibrosis, what is the probability that the second child will be normal?

Answers:

a.) draw the pedigree as far as described?  

Pedigree:

C/–      c/c

       C/c       C/–

              ?

b.) If the frequency in the population of heterozygotes for cystic fibrosis is 1 in 50, what is the chance the couples first child will have cystic fibrosis?  

Man: has the disease

Wife: 1/50 chance to have the c allele

First child: 1.0 x 1/50 x 1/4 = 1/200 = 0.005  

c.) If the first child does have cystic fibrosis, what is the probability that the second child will be normal?

If the first child has the disease, then the mother is a carrier of the

c allele. In consequence, the probability is 3/4

every el niño cycle is different. every year.