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#1
03-21-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,353 Threads: 94 Thanked 874 Times in 506 Posts Reputation: 887
NBME 12 question

Ok, I came across this HARDY-WEINBERG question in the 3rd section, where they ask you to figure out the carrier risk.
Man, I cant figure it out! I calculate the answer, but its not in the options list.

They have an incidence of CF in the general population to be 1/40,000

so i take a square root of it and get 1/200

trying to find the carrier rate (2pq)

I have 2 x 199/200 x 1/200

I get 0.0095 as a result!

Do they mean that this is kinda close to 1/100 or what? I have trouble with Hardy-Weinberg.
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#2
03-21-2012
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you are correct till 2pq = 200 ....... but have to take p as 1 so q = 1/100
#3
03-21-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,353 Threads: 94 Thanked 874 Times in 506 Posts Reputation: 887

Quote:
 Originally Posted by Hitman you are correct till 2pq = 200 ....... but have to take p as 1 so q = 1/100
But why do u take p as a 1, when its 1-q
(p+q=1)
?
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#4
03-21-2012
 USMLE Forums Master Steps History: Not yet Posts: 928 Threads: 17 Thanked 538 Times in 375 Posts Reputation: 548

Quote:
 Originally Posted by DocSikorski But why do u take p as a 1, when its 1-q (p+q=1) ?
I know but its given in kaplan biochem ..... dont remember much but had noted down that to calculate carriers need to take p as 1 becoz p are normal people and are very large part of the population and q is a very small part so they approximate it .something like that dont know the exact detail ... just need to cram this up for solving them it i felt .....
 The above post was thanked by: DocSikorski (03-21-2012)
#5
03-21-2012
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NBME form 12 block 3 #20

With these type of questions...I suggest that you write down what's given as you're reading the question

- autosomal recessive
- incidence of cystic fibrosis in the population: (1)/(40,000)
- what's the wife's risk for being a carrier?

this type of question is called "carrier prevalence" &/or "disease prevalence"

in this case, they are asking for carrier prevalence = "wife's risk"

this is the equation that you need to use for this type of question is called "carrier prevalence" or "disease prevalence"

2 + 2q + q^2

2q = carrier prevalence

q^2 = disease prevalence

they gave you disease prevalence in the question = incidence of cystic fibrosis in the population = q^2 = (1)/(40,000)

take the square root of [(1)/(40,000)] = 1/200 = q

what's the wife's risk for being a carrier? they want use to find "carrier prevalence"

"carrier prevalence" = 2q = (2)[1/200] = 1/100

ans. 1/100
 The above post was thanked by: DocSikorski (03-21-2012), maymed (10-19-2015), terminator (03-21-2012)
#6
03-21-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,353 Threads: 94 Thanked 874 Times in 506 Posts Reputation: 887

Quote:
 Originally Posted by USMLE16 With these type of questions...I suggest that you write down what's given as you're reading the question - autosomal recessive - incidence of cystic fibrosis in the population: (1)/(40,000) - what's the wife's risk for being a carrier? this type of question is called "carrier prevalence" &/or "disease prevalence" in this case, they are asking for carrier prevalence = "wife's risk" this is the equation that you need to use for this type of question is called "carrier prevalence" or "disease prevalence" 2 + 2q + q^2 2q = carrier prevalence q^2 = disease prevalence they gave you disease prevalence in the question = incidence of cystic fibrosis in the population = q^2 = (1)/(40,000) take the square root of [(1)/(40,000)] = 1/200 = q what's the wife's risk for being a carrier? they want use to find "carrier prevalence" "carrier prevalence" = 2q = (2)[1/200] = 1/100 ans. 1/100

But why do u take p as 1?
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#7
03-21-2012
 USMLE Forums Scout Steps History: Step 1 Only Posts: 40 Threads: 3 Thanked 25 Times in 20 Posts Reputation: 35

Quote:
 Originally Posted by DocSikorski But why do u take p as 1?
I think it's becoz value of P >>> q and it reaches approximately 1. So for simplicity p=1 is used.
#8
03-21-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,353 Threads: 94 Thanked 874 Times in 506 Posts Reputation: 887

Quote:
 Originally Posted by terminator I think it's becoz value of P >>> q and it reaches approximately 1. So for simplicity p=1 is used.
Well no. For example if the prevalence is 1/25 in the general population, than you definitely cannot take p as 1.

So I guess In this case .0095 is rounded to .01 = 1/100...
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#9
03-21-2012
 USMLE Forums Scout Steps History: Step 1 Only Posts: 40 Threads: 3 Thanked 25 Times in 20 Posts Reputation: 35

In that case, Yes you can't take p=1 becoz prevalence of the disease is very high vs in the question where it is very low. In any case, I would do it the old fashioned way. It's less complicated. lol
 The above post was thanked by: DocSikorski (03-21-2012)

 Tags Biostatistics-Epidemiology, NBME-

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