How do you get a sex chromosome? A little bird told me this...

What is sex?

Musings at 10,000 Birds:

This is a simple question with a complicated answer. Part of the answer is this: The biological identity of an individual that guides its choice of mate. So, in a simple version of the world of a bird, there are two sexes, male and female, and males chose females and females chose males as mates.

Assume for a moment that there is choice, and that the choice is based on a discernible feature. So, for example, males and females prefer to mate with a member of the opposite sex who has a blue and white pattern on its breast. So far so good, and so far simple. A bluer blue together with a whiter white on a female or male will be extra attractive to the member of the opposite sex.

But what if there emerged a genetically novel version of the males who lacked the white, but would only mate with females that lacked the blue. And, the obverse occurred as well. Suddenly you would have multiple sexes, beyond the usual two. There would be two kinds of males and two kinds of females.

Let’s ask the question again, what is sex? In a world in which the final adult outcome with respect to sex can be highly variable, one might look more deeply to find a simple binary observation to tell you if an individual is a male or a female. At the deepest level are presumably some genes, or maybe one gene, that matters, but there may be other equally important things that are not directly genetic as well. For example, in rats (as in “lab rat”) if the mother of a nominally (genetically) male offspring does not repeatedly lick the anogenital region of the pup, the usual cascade of hormones and hormone induced changes, involving androgens, will not occur in that individual and the final outcome will not be a rat that will mount and mate with a female.

Somewhere in between the gene and the anogenetical licking (or gender policing or incubation temperature or whatever else matters) is the sex chromosome. A sex chromosome may be a section of DNA (as a chromosome is) .... Read the rest here