Re: Question about avian sex chromosomes

[Mickey Mortimer <Mickey_Mortimer111@msn.com>]

Dora Smith wrote-

> As far as I know, all higher living things, let alone all higher animals,
> that reproduce sexually, have a female with two X chromosomes, and a male
> with one X chromosome and one Y chromosome.
>
> Do birds do sex chromosomes differently than mammals do?   If so, since
> dinosaurs and mammals share common ancestors, how could such a thing have
> happened?

A lot of different organisms have sex chromosomes (not just "higher"
organisms or "higher" animals), but it's not always like in mammals with XX
and XY.  Some plants, insects (eg. Drosophila), some lizards and turtles are
like mammals, with the male having the heterogenic chromosomes.  But in
birds, snakes, some lizards, some amphibians and other insects (eg.
lepidopterans), the females are heterogenous.  Some apmhibians and most fish
lack sex chromosomes.  As you can see, birds developed their system
separately from mammals.  I'm pretty sure their most recent common ancestor
lacked sex chromosomes of any sort.

Mickey Mortimer
Undergraduate, Earth and Space Sciences
University of Washington
The Theropod Database - http://students.washington.edu/eoraptor/Home.html