








USMLE Step 1 Forum USMLE Step 1 Discussion Forum: Let's talk about anything related to USMLE Step 1 exam 

Thread Tools  Search this Thread  Display Modes 
#1




HardyWeinberg Carrier of AR diseases?
If the question asks about the carrier of an autosomal recessive disease, do we use 2pq or pq?
e.g. if disease X (which we know is AR) has an incidence of 1/625, what is the carrier? And why? 
#2




Quote:
If the disease is AR and homozygous gene then you should use q2 but heterozygous gene use 2pq. Chances of inheriting a trait: Disease 25%, Carrier 67%. One child is born with two normal genes (normal) Two children are born with one normal and one abnormal gene (carriers, without disease) One child is born with two abnormal genes (at risk for the disease) 
#3




So what is the carrier? is pq or 2pq? Do you divide by 2 to get the carrier?



#4




Quote:
p2+2pq+q2=1 Frequency of normal allele = p Frequency of mutant allele = q Frequency of homozygous normal = p2 Frequency of heterozygous carriers = 2pq Frequency of homozygous affected = q2 Lets do an example using above equation: PKU Prevalence of PKU is 1 in 10 000 live births. What is the carrier frequency? q2 = 1/10000 want to calculate 2pq we know if we take the square of q q = 1/100 or 0.01 P = 10.01 = 0.99 which is close to 1 Since p is approximately 1, then Carrier frequency = 2q = 2(1/100) =1/50 = 0.02 Which means in this population 1 in every 50 people will carry this gene. I hope this may help you clear your doubt. 
#5




To simplify things for Step 1 exam;

The above post was thanked by:  
#6




Many thanks for your help, guys, But that last point confused me:
Quote:
So for example, if incidence of an AR disease is 1/100. then: Carrier = 2pq = 2 x 1 x 1/100 = 1/50 Is that right? 
#7




Also, why in AD do we consider affected = q`2? isn't it q`2 + 2pq?

#8




Quote:
Incidence (or prevalence) of an AR disease is q^2. Thus, q^2 here = 1/100. In order to get q, you will need to get the root of 1/100, which equals to 0.1 (1/10). Then, carriers = 2pq = 2 x 1 x 0.1 = 0.2 (1/5). 
#9




In AD diseases, frequency of affected persons is simply equal to carrier frequency, since all carriers are also affected. Thus, 2pq or 2q (since p is approx. = 1) is used for both frequencies of carriers and affected persons.

#10




Well that clears things, thanks a lot

#11




No problem. Please refer to my post #5 above, and just memorize it asis! It helped me a lot in the exam and in Qbanks. Just don't forget to get the root when given an incidence/prevalence of an AR disease or an XR disease in females (since both are = q^2) if the question is asking about the carrier prevalence (2q).

#12




Many thanks, and if you have any idea about my other question:
http://www.usmleforums.com/usmlest...enetrance.html Please tell me what you know, genetics is a bit cruel to me.. 
Tags 
Genetics 
Thread Tools  Search this Thread 
Display Modes  

Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Trinucleotide Repeats Diseases  ashishkabir  USMLE Step 1 Bits & Pieces  5  06282017 11:35 PM 
X Linked Dominant Diseases  MLELove  USMLE Step 1 Bits & Pieces  1  08282012 07:11 PM 
Hardy Weinberg Equation, Please Help  achistikbenny  USMLE Step 1 Forum  6  07032010 12:56 AM 
Hardy weinberg equation!!!  fauzan  USMLE Step 1 Forum  7  12302009 12:23 PM 
Paget Diseases  StepTaker  USMLE Step 1 Bits & Pieces  0  09032009 03:23 PM 
