In X linked lethal diseases,
those who are affected don't go on to reproduce.
In the absence of fam hx...
...the prior probability of the
mother (of an affected child)
being a carrier is 2/3.
...the prior probability of the
mother being a non carrier is 1/3. This would mean the disease is de novo.
...the prior probability of the
grandmother (of an affected child)
being a carrier is 1/3.
...the prior probability of the
aunt of an affected child,
being a carrier is 1/6.