Advanced Hardy-Weinberg Problem Solving: Sex-Linked Inheritance
Hardy-Weinberg equilibrium was reviewed in the last article, linked here. Let's look into another variation of H-W problem-solving: investigating sex-linked traits at a population level.
The first thing to note is that in any human population, males are hemizygous for X-linked traits. Hemizygous means that they only have one locus for X-linked alleles (because males are XY - there's only one place for X-linked traits to be. This is in contrast to females, who are XX - they thus have two places for X-linked traits to be, and thus can be either homozygous - both the same - or heterozygous - both different.)
Because human males only have one spot for X-linked traits to be, the proportion of males who express one variant of the trait versus another is equal to the allele frequencies themselves. In other words, if a population of males is 60% XAY and 40% XaY, then the allele frequency of A has to be 60%, and the allele frequency of a has to be 40%. The classic p^2+2pq+q^2 does not apply because there is no homozygous / heterozygous, so the p^2/q^2 and 2pq are not relevant. It's just p+q=1: the equation for allele frequencies is numerically the same as the equation for genotypes.
You can use this in problems to find out the allele frequency from info given about males. You can use this allele frequency to calculate things for females (we assume the male and female allele frequencies are the same). I've linked two video explanations of how to do this at the end of the article.
There's another cool thing that's tangentially relevant, but if I ever qualify to write USABO questions, I'll definitely do a little mixing in of this topic. What if, instead of human sex linkage, we looked at chickens? Chickens have a ZZ/ZW system, so what if we looked at Z-linked traits? It's essentially the same thought process as described above, just with sexes switched (in chickens, the females would be hemizygous for Z-linked traits). There's more sex chromosome systems, and these could make cool H-W problems, too! Here's a few relevant chromosomal sex determination systems (image edited bc sus):
Yeah. Here's the two videos walking you through the process:
