Hossein
Forum Newbie
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this q is from kaplan genetics lecture notes:
"A 30-year-old man is phenotypically normal, but two of his siblings died from infantileTay-Sachs disease, an autosomal recessivecondition that is lethal by the age of five.Whatis the risk that this man is a heterozygous carrier of the disease-causing mutation?
A.1/4 B.1/2 c.2/3 D.3/4 E.not elevated above that of the general population
Answer: C. Because two of the man's siblings had Tay-Sachs disease, his parents must both be carriers. This clearly elevates his risk above the general population and excludes choiceE. He is not affected, so this excludes choice A, which is the probability of inheriting two copies of the disease allele. His risk of inheriting one copy of the disease gene at conceptionis 1/2 (choice B). However, the fact that he is phenotypically normal at age 30 means that he cannot have inherited copies of the disease gene from both parents. Only three possibilities remain: Either he inherited no copies of the mutation, he inherited a copy from his father, or he inherited a copy from his mother. Each of these three possibilities is equally likely, and two of them lead to heterozygosity. Thus, the risk that he is a carrier is 2/3."
I don't get it. Shouldn't it be 1/2 cos the probability of receiving the carrier genotype is 1/2 at any conception?
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| study_ing
Forum Fanatic
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make the punnet square.
the overall risk for a given child is 2/4= 1/2
this guy is still normal at 30. this means he surely isnt a homozygote for the diseased allele.
cross ot that option from the punnet square.
now ull c that the probability for this HEALTHY guy of being a hetrozygote is 2/3
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| fox
Forum Guru
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whats a punnet square, sorry I still havent started with genetics
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| Hossein
Forum Newbie
Topics: 5
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thanks,all clear now!
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