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#1
11-28-2012
 USMLE Forums Veteran Steps History: 1+CK+CS Posts: 259 Threads: 8 Thanked 115 Times in 78 Posts Reputation: 125
Hardy Weinberg Genetics

Clearly I fell asleep in genetics when we learned this. now its coming back to haunt me. IM lost and would like someone to teach me how to solve questions like these. PLEASE

In a small city with a stable Caucasian population, the carrier frequency of cystic fibrosis is 1/30. In a nearby community, the cystic fibrosis carrier frequency in Asians is 1/100. What is the probability that a child born to a mother from the Caucasian community and a father from the Asian community will suffer from the disease?
 The above post was thanked by: neha_subh (04-05-2013)

#2
11-28-2012
 USMLE Forums Newbie Steps History: 1+CK+CS Posts: 6 Threads: 2 Thanked 8 Times in 2 Posts Reputation: 18

For probability questions you have to multiply....
Since cystic fibrosis is an Autosomal Recessive condition, both parents must carry the allele to pass it on to the next generation.
If either parent is a carrier, the probability that the child will inherit the mutant allele is 1/2.
So

Mother= 1/2 * 1/30 Father= 1/100 * 1/2
1/60 * 1/200

 The above post was thanked by: docusmle9 (04-04-2013), gen88 (11-29-2012), mdsoon (11-28-2012), neha_subh (04-05-2013), tootsie (11-29-2012)
#3
04-04-2013
 USMLE Forums Newbie Steps History: Not yet Posts: 2 Threads: 0 Thanked 0 Times in 0 Posts Reputation: 10

Mmm.. doesn't this answer only assume if the parents were only carriers? Don't we have to account for if the homozygous recessive parents were to meet?

ie.
Carrier = 1/30 = 2pq
Homozyg Recess = 1/60

Then you have to calculate total probability of a Carrier from the caucasian community, with a carrier from asian, or with a homozygous recessive from asian, and vv?

I feel like we completely ignored the homozygous recessive.

Can someone double check me?

#4
04-05-2013
 USMLE Forums Scout Steps History: Not yet Posts: 44 Threads: 9 Thanked 6 Times in 5 Posts Reputation: 16

chances of mother being carrier = 1/ 30
chances of father being carrrier =1/100
chances of 2 carrier mating n produciing affected child = 1/4
therefore chances of child being affected is = 1/4* 1/30* 1/100 = 1/12000
 The above post was thanked by: neha_subh (04-05-2013), roopa.loveurlife (05-24-2013)
#5
04-05-2013
 USMLE Forums Newbie Steps History: Not yet Posts: 2 Threads: 0 Thanked 0 Times in 0 Posts Reputation: 10

Again... what about the homozygous recessive? Is this kind of a question assuming that both of the parents are only carriers, and can't be homozygous recessive?

Carrier frequency is only telling us 2pq (heterozygous) and nothing about q2 isn't it?

Thanks!
#6
04-05-2013
 USMLE Forums Scout Steps History: 1 + CK Posts: 40 Threads: 4 Thanked 22 Times in 14 Posts Reputation: 32

Quote:
 Originally Posted by Jun1023 Again... what about the homozygous recessive? Is this kind of a question assuming that both of the parents are only carriers, and can't be homozygous recessive? Carrier frequency is only telling us 2pq (heterozygous) and nothing about q2 isn't it? Thanks!
If they are homozygous recessive,then they clearly mention thier phenotype(affected with CF)..In the above question,they didnt mention any such,so we hav to consider they are phenotypically normal(can be homozygous for normal allele or heterozygous for mutant alelle ). Thus u calculate carriers who appear phenotypically normal and their chance of producing affected child.
 The above post was thanked by: Jun1023 (04-05-2013), neha_subh (04-05-2013)
#7
04-05-2013
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 824 Threads: 66 Thanked 463 Times in 298 Posts Reputation: 473

Quote:
 Originally Posted by Jun1023 Again... what about the homozygous recessive? Is this kind of a question assuming that both of the parents are only carriers, and can't be homozygous recessive? Carrier frequency is only telling us 2pq (heterozygous) and nothing about q2 isn't it? Thanks!
question is about carriers or 2pq
that is why u multiply the carrier frequency by 1/2=chance of carrier mom or dad giving affected gene

if it was q2 or (disease frequency)-than there would be no need for multiplication by 1/2
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