Step I biostats question from NBME5

[cattyjane]

carrier frequency is 1/30. What's the frequency of autosomal recessive offspring? How do you do the calculation?

1/30*1/30?

 

[Vikingwizardguy]

Not sure but I'll give it a go...

if carrier frequency is 1/30 that means:

2pq = 1/30
pq = 1/60
assume p is approximately 1 then q = 1/60 leaving the frequency of autosomal recessive offspring to be (1/60)^2 = 1/3600.

My problem is with the assumption of p being about 1. Remember p = 1-q and generally q is a very small fraction of the whole (since it's the frequency of finding the autosomal allele in the population and in this case it would be about 1 in 60)

 

[MilkmanAl]

Another way to do it is to use the fact that there's a 1/30*1/30 probability that two carriers do the big nasty and a 1/4 chance that their kid gets both recessive alleles. that's a tad less number crunching for the less numerically inclined.

 

[Vikingwizardguy]

Another way to do it is to use the fact that there's a 1/30*1/30 probability that two carriers do the big nasty and a 1/4 chance that their kid gets both recessive alleles. that's a tad less number crunching for the less numerically inclined.

I prefer your method, it does not require a vague assumption.

 

[cattyjane]

thanks guys, I like both of the answers