A young couple went to see a genetic counselor because each had a sibling affect

Question

A young couple went to see a genetic counselor because each had a sibling affected with cystic fibrosis. (Cystic fibrosis is a recessive disease, and neither member of the couple nor any of their four parents is affected.)

a. What is the probability that the female of this couple is a carrier?
b. What are the chances that their child will be affected with cystic fibrosis?
c. What is the probability that their child will be a carrier of the cystic fibrosis mutation?

Please show work as I already have the answers.

Explanation / Answer

(a)If the female has a sibling with CF then her parents must both be heterozygous CFcf x CFcf = .3 + .3 = 2/3 1:CFCF: 2:CFcf: 1 cfcf (b) What are the chances that their child will be affected with CF? The change that either parent is a carrier is 2/3. Thus the chances of having a child with genotype cf/cf is 2/3 X 2/3 X 1/4 = 4/36 = 1/9. (c) What is the probability that their child will be a carrier of the CF mutation? The child would be a carrier if its genotype were Cf/cf (or cf/+). This could occur if both parents were heterozygotes: 2/3 X 2/3 X 1/2 = 4/18 = 2/9 Or if one parent is heterozygous: 2/3 X 1/3 X 1/2 = 2/18 = 1/9 + 1/3 X 2/3 X 1/2 = 1/9 = 2/9 Thus the probability is 2/9 + 2/9 = 4/9.

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