[生物] 想請問一題普生

[yangboxx (鳥)]

14. John and Mary are planning to start a family. John was married once
before, and he and his first wife had a child with cystic
fibrosis. The brother of his current wife, Mary, died of cystic fibrosis.
What is the probability that John and Mary will have a
baby with cystic fibrosis? (Neither John, Mary, nor their parents have cystic
fibrosis.)
A) 1/4 B) 1/6 C) 1/8 D) 1/12 E) 1/16

答案是B

我的想法是
John是Tt，Mary是TT或Tt，所以不是1/2×1/4=1/8嗎!?

先謝謝各位大大~~

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推 BiGiLa: Mary TT/Tt機率並不是一半一半喔 TT1/3 Tt2/3
→ BiGiLa: 1/2 X( 2/3 X 1/2 ) = 1/6
→ yangboxx: 對喔!!謝謝大大~~