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#1
05-16-2011
 USMLE Forums Guru Steps History: Step 1 Only Posts: 343 Threads: 127 Thanked 525 Times in 192 Posts Reputation: 535
The probability of getting an affected homozygous child?

A man is known heterozygous carrier of a mutation causing hyperprolinemia,and autosomal recessive condition. Phenotypic expression is variable and ranges from high level of excretion of proline to neurological signs and symptoms. Suppose that 0.0025% (1/40,000) of population is homozygous for the mutation causing this disease. If the man mates with somebody from population, what is the probability that he and his mate will produce a child who is homozygous for this condition..

a) 1% (1/100)
b) 0.5%(1/200)
c) 0.25%(1/400)
d) 0.1%(1/1000)

#2
05-16-2011
 USMLE Forums Newbie Steps History: Not yet Posts: 2 Threads: 1 Thanked 0 Times in 0 Posts Reputation: 10

I think c (0.25%)
homozygotes(q^2)=1/40000, then q=1/200
heterozygote will be 2pq=2x1x1/200=0.01
the chance of produce homozygote child if two parents were hetrozygotes is 0.25
so 0.25x0.01=0.25%

 Tags Genetics-, Step-1-Questions

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