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Just started with Genetics (Kaplans)n came around this question:

A 30 yr old man is phenotypically normal,but two of his siblings died from infantile Tay-sachs disease,an autosomal recessive condtion that is lethal by age of 5.What is the risk that this man is heterozygous carrier of the disese causing mutation?
A) 1/4
B) 1/2
C) 2/3
D) 3/4

Why the answer is C ,why not B .....:confused:
 

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Just started with Genetics (Kaplans)n came around this question:

A 30 yr old man is phenotypically normal,but two of his siblings died from infantile Tay-sachs disease,an autosomal recessive condtion that is lethal by age of 5.What is the risk that this man is heterozygous carrier of the disese causing mutation?
A) 1/4
B) 1/2
C) 2/3
D) 3/4

Why the answer is C ,why not B .....:confused:
Well, he's phenotypically normal, so we know that he does not have both T-S alleles. That leaves three options: either he has two normal alleles, one normal from dad and one T-S from mom, or one T-S from dad and one normal from mom. In two of those three scenarios, he is a heterozygous carrier, so 2/3.
 
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