USMLE Genetics Question Help

[teenup91]

The incidence of PKU in a population is 1 in 6,400 individuals. A woman from this population marries a phenotypically normal man who had an uncle with PKU. What is the risk that this couple will have a child with PKU?

1) 1/480
2) 1/6,400
3) 1/80
4) 1/40
5) 1/3,200

Woman's risk: 1/40
Man's risk: ???? (I think ?)
Mendelian: (¼)

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[W19]

I hate these genetics problems...

p^2 = 1/6,400 ; therefore the woman has 1/80 chance of having PKU allele...

If the guy has an uncle with PKU, we can assume his dad or mom has 2/3 of chance of carrying the allele (punnett square) . So the mom or dad has 1/4 of a chance to pass on the allele to him : (2/3)x(1/4) = 1/6

(1/80)(1/6) = 1/480

Hope I am right..