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.