Question

A man (Albert) and a woman (Beth) do not have cystic fibrosis and they have a daughter (Celia) with cystic fibrosis and a son (David) who does not have cystic fibrosis. The daughter grows up and she mates with a male (Evan) who does not have cystic fibrosis. Their only child is a boy (Frank) and he has cystic fibrosis. (You do not need to include a pedigree of this situation but I strongly recommend you draw one for your own understanding).

c. If Celia and Evan have another child what is the probability that it will be a boy and have cystic fibrosis? Explain your answer.

d. What is the probability that Celia and Evan’s first daughter will have cystic fibrosis? Explain your answer.

Answer 1

Answer- cystic fibrosis is an autosomal recessive disorder.

So for an individual to be affected both alleles must be recessive so homozygous recessive genotype.

Ans

Parents - celia × Evan

- cc × Cc

	C	c
c	Cc	cc
c	Cc	cc

In every pregnancy there is a 50% chance of being either a boy or a girl and 50% chance the offspring will have cystic fibrosis .

So 0.5×0.5 = 0.25 = 25% chance the next son will be a boy with cystic fibrosis.

D- 50%

In each pregnancy there is 50% chance of the offspring having cystic fibrosis so there is 50% chance their first daughter will have cystic fibrosis and 50% chance will be unaffected. That is 50% chance Evans will give recessive c allele because celia is homozygous recessive so she can only give c allele .

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