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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%.If the question asks about thecarrierof an autosomal recessive disease, do we use2pqorpq?

e.g. if disease X(which we know is AR)has an incidence of 1/625, what is the carrier? And why?

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)

Autosomal Recessive DiseasesSo what is the carrier? is pq or 2pq? Do you divide by 2 to get the carrier?

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 = 1-0.01 = 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.

Carriers ofanydisease: 2pq

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?

No, this is wrong.Many thanks for your help, guys, But that last point confused me:

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?

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

In AD diseases, frequency of affected persons is simply equal to carrier frequency, since all carriers are also affected. Thus, 2pq orAlso, why in AD do we consider affected =q`2? isn't itq`2 + 2pq?

No problem. Please refer to my post #5 above, and just memorize itWell that clears things, thanks a lot

http://www.usmle-forums.com/usmle-step-1-forum/4976-complete-penetrance.html

Please tell me what you know, genetics is a bit cruel to me..

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