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#1
02-19-2010
 USMLE Forums Scout Steps History: Not yet Posts: 33 Threads: 30 Thanked 60 Times in 24 Posts Reputation: 70
Genetic probability Question

Prevalence of homozygotes in a population is 1%. If a woman is a known carrier marries somebody from that population then what is the probability that they'll produce an affected homozygous child?

A- 0.09
B- 0.07
C- 0.05
D- 0.025
E- 0.25
F- 0.50
G- 0.10
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#2
02-19-2010
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Anybody knows the answer to that
#3
02-19-2010
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i think the answer is E

#4
02-19-2010
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Quote:
 Originally Posted by Abdi i think the answer is E
Please tell me how did u arrive to this conclusion!
#5
02-19-2010
 USMLE Forums Scout Steps History: 1+CK+CS Posts: 51 Threads: 3 Thanked 30 Times in 19 Posts Reputation: 40

The woman has three possibilities:
married to affected homozygous 1%
married to affected heterozygous (should be calculated by Hardy weinburg)
married to homozygous normal person (the remaining possibility)

These probabilities should be combined

If some one can do the math for us to know the answer
#6
06-07-2010
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I know this has been up for a while and has not been answered. I would like to know what you guys think...

I think the answer is C
#7
06-07-2010
 USMLE Forums Guru Steps History: --- Posts: 487 Threads: 69 Thanked 341 Times in 163 Posts Reputation: 361
C

its C 0.05
disease prevalence 1% = 1/100 = q2
allele frequency = q = 1/10
when disease frequency is less then 1/100 p(normal allele frequency) is considered 1
so....carrier frequency will be 2pq = 2q = 1/5
for AR disease , father must be carrier to transmit disease to child...
probability that women will marry carrier man is 1/5
probability that women will transmit her diseased allele to child is 1/2
probability that man will transmit his diseased allele to child is 1/2
now just multiply 1/5*1/2*1/2 = 1/20 = 0.05
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#8
06-07-2010
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Oh okay sweet.
I did it a bit differently as I included the likelyhood of marrying a homozygous ressessive and having a child (ended up being .005)
I also didn't assume p as being 1 (rather .9) so i got .045 instead of .05
The grand total was .05.
I guess either way works.
#9
10-07-2013
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Quote:
 Originally Posted by khushboo its C 0.05 disease prevalence 1% = 1/100 = q2 allele frequency = q = 1/10 when disease frequency is less then 1/100 p(normal allele frequency) is considered 1 so....carrier frequency will be 2pq = 2q = 1/5 for AR disease , father must be carrier to transmit disease to child... probability that women will marry carrier man is 1/5 probability that women will transmit her diseased allele to child is 1/2 probability that man will transmit his diseased allele to child is 1/2 now just multiply 1/5*1/2*1/2 = 1/20 = 0.05

I have a doubt..is the probability of man transmitting the dz is 1/4 coz the question says the woman will marry a guy from homozygous population??

Can anybody explain this question again?
#10
10-08-2013
 USMLE Forums Scout Steps History: 1+CK+CS+3 Posts: 97 Threads: 16 Thanked 64 Times in 23 Posts Reputation: 74
my solution

may i suggest another solution

since 1% of that population is homozygous normal or diseased
if the woman marries a homozygous normal from that 1% she wont get a diseased homozygous baby
if she marries a homozygous diseased person from that same 1% she will get a homozygous diseased baby but for our calculation purpose we are going to neglect that homozygous diseased population since it will be negligible as it is an AR disease and it will be a small fraction of that 1%

so the probabability of that woman marrying a heterozygous carrier is 99% or 0.9
the probability of her having a homozygous diseased baby if she marries a heterozygous carrier = 1/4 or 0.25
the probability of husband having a homozygous diseased baby if he marries a heterozygous carrier = 1/4 or 0.25
multiply all the probabilities
0.9 x 0.25 x 0.25 = 0.05
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#11
10-09-2013
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#12
10-12-2013
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I think answer is 0.25or e. since woman gonna merrie man from that 1% population of homozygous , it's 1/2 chance that that man is affected homozygous and woman transfers her affected allele is 1/2, so calculating 1/2 times 1/2=0.25
#13
10-12-2013
 USMLE Forums Veteran Steps History: Step 1 Only Posts: 215 Threads: 23 Thanked 185 Times in 75 Posts Reputation: 195

The question is asking you this -

Probability of a homozygous child if someone "from the population" marries a known carrier female. Let's call her Miss X.

Now by population, it means 2 possibilities -

Situation 1 - Another carrier from the population marries Miss X. The probability of this event happening is - 2pq = 2x0.9x0.1 = 0.18 (Note: the square of q = 0.01, hence 1=0.1 and p=1-0.1 = 0.9. We can't ignore p in the equation because the prevalence is high.)
Now if this mating occurs, probability of having a homozygous child = 0.25 (draw a punnet square if you please.)
Hence combining the probabilities, final probability from situation 1 = 0.18x0.25 =0.45.

Situation 2 -A homozygous diseased male marries Miss X. Note that the question doesn't suggest that the female will only marry a phenotypically normal guy. The probability of this event happening is 0.01 (frequency of homozygotes is given in the question as 1%).
If this mating occurs, probability of having a homozygous child = 0.5.
Combined probability from situation 2 = 0.05.

Now to calculate the final answer, we must understand that EITHER of the 2 situations would give us our homozygous child. In these scenarios, the probabilities are to be added (not multiplied.)

So combining situation 1 and 2, final probability = 0.45 + 0.05 = 0.5

I know this is confusing and we probably won't be tested such complicated applications but once you read it slowly and refer to your genetics book, you would get it. I think a similar questions can be found in the Kaplan qbank. Hope that helped.
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