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#1
05-15-2011
 USMLE Forums Guru Steps History: Step 1 Only Posts: 343 Threads: 127 Thanked 522 Times in 192 Posts Reputation: 532
Calculating the "P" in Hardy-Weinberg Equation

here is question about PKU relating to following equation

p2 + 2pq + q2 = 1

prevalence of PKU is 1 in 10,000 live births
WHAT IS CARRIER FREQUENCY
here

q2= 1/10,000
we want to calculate 2pq

q= 1/100 =0.01
HOW WE CAN CALCULATE p??
prof says p would be 1-q
how p would be 1-q following above equation?

#2
05-16-2011
 USMLE Forums Addict Steps History: 1+CK+CS Posts: 189 Threads: 20 Thanked 506 Times in 99 Posts Reputation: 516

Second part to the equation is P+q=1
so thts how u get p=1-q
#3
05-16-2011
 USMLE Forums Guru Steps History: Step 1 Only Posts: 343 Threads: 127 Thanked 522 Times in 192 Posts Reputation: 532

Quote:
 Originally Posted by mayursn39 Second part to the equation is P+q=1 so thts how u get p=1-q
can u explain plz how p+q=1?
i didnt find second part of equation in book
#4
05-16-2011
 USMLE Forums Addict Steps History: 1+CK+CS Posts: 189 Threads: 20 Thanked 506 Times in 99 Posts Reputation: 516

P+q=1 is as follows

p=frequency of normal allele
q= frequency of disease allele

So if we add these both we get total allele frequency irrespective of diseased or not which represents entire population.
reference
http://anthro.palomar.edu/synthetic/synth_2.htm
 The above post was thanked by: INCOGNITO (05-16-2011)
#5
05-16-2011
 USMLE Forums Guru Steps History: Step 1 Only Posts: 343 Threads: 127 Thanked 522 Times in 192 Posts Reputation: 532

Quote:
 Originally Posted by mayursn39 P+q=1 is as follows p=frequency of normal allele q= frequency of disease allele So if we add these both we get total allele frequency irrespective of diseased or not which represents entire population. reference http://anthro.palomar.edu/synthetic/synth_2.htm
wooo
great idea
its deduction of equation from main stem equation which is
p+q=1
taking square of it

p2+2pq+q2=1

thanx a lot
#6
05-16-2011
 USMLE Forums Veteran Steps History: Not yet Posts: 236 Threads: 4 Thanked 123 Times in 95 Posts Reputation: 133

Quote:
 Originally Posted by IKR123 here is question about PKU relating to following equation p2 + 2pq + q2 = 1 prevalence of PKU is 1 in 10,000 live births WHAT IS CARRIER FREQUENCY here q2= 1/10,000 we want to calculate 2pq q= 1/100 =0.01 HOW WE CAN CALCULATE p?? prof says p would be 1-q how p would be 1-q following above equation?

#7
05-16-2011
 USMLE Forums Guru Steps History: Step 1 Only Posts: 343 Threads: 127 Thanked 522 Times in 192 Posts Reputation: 532

Quote:
 Originally Posted by noothan Is the answer 1/25 ?
no dude :O

#8
05-16-2011
 USMLE Forums Veteran Steps History: Not yet Posts: 236 Threads: 4 Thanked 123 Times in 95 Posts Reputation: 133

From goljan

Prevalence rate = [carrier rate/2] raised to 2
#9
05-16-2011
 USMLE Forums Guru Steps History: Step 1 Only Posts: 343 Threads: 127 Thanked 522 Times in 192 Posts Reputation: 532

Quote:
 Originally Posted by noothan yeah my bad From goljan Prevalence rate = [carrier rate/2] raised to 2
goljan says so?
then it does not fit this scenerio...
how to explain according to goljan?
#10
05-16-2011
 USMLE Forums Veteran Steps History: Not yet Posts: 236 Threads: 4 Thanked 123 Times in 95 Posts Reputation: 133

yeah
RR path goljan page 82 third editon

He discuss how to calculate prevalence if carrier rate is given .i just posted as i saw prevalence and carrier rate,so posted ..........so it cant be used here or is this forumula only for AD ?

Last edited by noothan; 05-16-2011 at 11:20 AM. Reason: error
#11
05-28-2011
 USMLE Forums Addict Steps History: 1 + CS Posts: 113 Threads: 8 Thanked 88 Times in 61 Posts Reputation: 98

Quote:
 Originally Posted by IKR123 here is question about PKU relating to following equation p2 + 2pq + q2 = 1 prevalence of PKU is 1 in 10,000 live births WHAT IS CARRIER FREQUENCY here q2= 1/10,000 we want to calculate 2pq q= 1/100 =0.01 HOW WE CAN CALCULATE p?? prof says p would be 1-q how p would be 1-q following above equation?
P^2 + 2Pq + q^2 =1

q^ = 1/10,000 => q = 1/100

then,

P^2 + 2P*(1/100) + 1/10,000 = 1

in order to get 1, since p and p^2 are too small, P must approach to 1...

finally:

CARRIER FREQUENCY = 2Pq = 2*1*(1/100) = 2/100 = 1/50

I hope this help

Last edited by Claus_CU; 05-28-2011 at 11:29 AM.
 The above post was thanked by: Dr. Mexito (03-24-2012), LatinGeorge (12-19-2011)
#12
10-15-2011
 USMLE Forums Newbie Steps History: Not yet Posts: 7 Threads: 0 Thanked 7 Times in 4 Posts Reputation: 17

The way I remember this for the exam is: The carrier frequency = 2 x the square root of the prevalence of the disease.

The assumption that this makes (and is generally true) is that the prevalence of the non-diseased approximates 1.

That is why 2pq is generally just 2 p since the q can be dropped as it approximates 1.
#13
03-24-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 662 Threads: 72 Thanked 363 Times in 202 Posts Reputation: 373

Quote:
 Originally Posted by Claus_CU P^2 + 2Pq + q^2 =1 q^ = 1/10,000 => q = 1/100 then, P^2 + 2P*(1/100) + 1/10,000 = 1 in order to get 1, since p and p^2 are too small, P must approach to 1... finally: CARRIER FREQUENCY = 2Pq = 2*1*(1/100) = 2/100 = 1/50 I hope this help
So... we first multiply (P*q) and then we multiply that result by 2??? Is that correct?
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"Disease is very old, and nothing about it has changed. It is we who change as we learn to recognize what was formerly imperceptible." JMC
#14
03-25-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,353 Threads: 94 Thanked 873 Times in 506 Posts Reputation: 886

If q is 1/1,000,000 or whatever than p is 999,999/1,000,000 no matter how low is q, bare in mind that p is always 1-q. So carrier frequency will be 2x(1-q)xq you can choose the closest answer option to your value. Practice this and then when u see the logic u can assume the p is 1 or very close to it when q is extremely low.

But in cas of CF let's say in Norway, q will be around 1/5...
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Ambition is a dream with a V8 engine. EP
 The above post was thanked by: Dr. Mexito (03-25-2012)

 Tags Biostatistics-Epidemiology, Genetics-

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