I'm looking at the following question
Im interested in the part where it says "Suppose Judy, a brown eyed child of brown-eyed parents, ... Find the probability that Judy is a heterozygote."
Here are the solutions with the previous answers displayed too.
Now i'm not sure this solution is technically correct. The pieces of information they seem to have left out of the probability calculation is that the parents have brown eyes and that the partner she marries is a heterozygote. Is the reason they have left those pieces of information out because the problem would much more difficult to model ?
I think they should have calculated
$$P(\text{(parents have brown eyes)} \cap \text{(Judy marries a heterozygote) } \cap (n \text{ children have brown eyes})|\text{Judy is Xx})$$
but what it seems they calculate is
$$P((n \text{ children have brown eyes}|\text{Judy is Xx } \cap \text{Judy marries a heterozygote})) = \left(\frac{3}{4}\right)^n$$
The calculation is correct.
That Judy married a heterozygote is just a fact that's given to us; it's not considered to be related to the random variables. Of course theoretically there could be such a relationship (e.g. heterozygotes could produce similar pheromones and tend to mate more often), but none is assumed here.
That the parents have brown eyes was taken into account in the first part of the solution that yielded $2p/(1+2p)$. It's included in “all previous information” and taken into account by using $2p/(1+2p)$ as the new a priori probability for Judy to be a heterozygote.