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Prep4USMLE Forum » USMLE Step 1 Forum » Biochemistry Forum

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4 Posts

Rachana
Forum Junior

Topics: 23
Posts: 52

06/02/04 - 08:25 AM

30 yr old man is phenotypically normal. his 2 siblings died from Tay sachs an AR disese that is lethal by age 5. what is the risk that the man is hyterozygous for the disease causing mutation?
1. 1/4
2. 1/2
3. 2/3
4. 3/4
5. not elevated above that of general population.

This is a Q bank question. But I am not satisfied with the explanation and answer. can some one give an answer and explain please?

mash
Forum Fanatic

Topics: 147
Posts: 1,326

06/02/04 - 08:33 AM

ANS IS 2/3
its an AR ds
his 2 sibs died frm taysachs => both his parents are heterozygous fr mutant allele
then 4 possible combs wud be AA, Aa,Aa and aa
coz he does not ve tay sachs we can ignore aa(==> he is either carrier or normal)
we r left with AA, Aa andAa
risk of being a carrier wud be 2/3

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kalsam
Forum Elite

Topics: 31
Posts: 361

06/02/04 - 02:37 PM

Yes it is 2/3..good explanation mash!!!

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tasneembanu
Forum Guru

Topics: 96
Posts: 545

06/24/04 - 03:59 PM

actually when i was doin this question,i dint understand the explanation,but now i do,ur a genius.

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