Re: Question about avian sex chromosomes

[David Marjanovic <david.marjanovic@gmx.at>]

> How do fish do being male and female without sex chromosomes?

Most genes that _really_ switch on the growth and differentiation of the
gonads are on completely different chromosomes anyway -- some important ones
are on chromosome 6 in humans, for example. Mammals have just evolved a
master switch that controls all those. If you have what's called sry, the
"sex-determining region on the Y chromosome", the genes for masculinity all
over the genome are switched on, otherwise you take the default
developmentary pathway = becoming female. Now different organisms have
different master switches. I guess birds have theirs on the female
chromosome... which is called either W or Z, I forgot. Crocodiles, turtles
and those lizards I've heard about so far use temperature as the master
switch; the temperature in the egg determines the sex. Fish? No idea. But
there are species that can change sex based on how many conspecifics of each
sex are around, so pheromones are to suspect as the master switch in these
cases.