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NBME 18 Answers

nbme18/Block 3/Question#33

4 yo boy, clumsy gait for 1 year

1/4

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submitted by sugaplum(51),

FA 2019 pg 59-61
X linked muscular dystrophy. The chance that the mom III-1 is a carrier is 1/2. XY (normal grandpa II-1) * XaX (carrier grandmom II-2) You know that III-1 is a female, so only look at half, so 1/2 chance.
The chance that III-1's mate is a carrier is near zero. So we make him normal. XY (normal Dad) * XaX (carrier mom III-1) Since we know its a boy you only look at half, so the chance of him having it is 1/2.
so 1/2 * 1/2= 1/4
These are two independent events, the chance the mom is a carrier * the chance the kid gets the affected X.