The gender of
both
mammals and birds is determined chromosomally. However,
whereas male
mammals are XY
(heterogametic) and
females XX
(homogametic), in
birds it is
the females that are
heterogametic (WZ) and males are homogametic (ZZ) [below, left]. As
in mammals,
this makes a convenient test for geneder possible. In
many
species of seabirds, the sexes are not strongly
differentiated morphologically. Characterization of the
sex-chromosome-specific gene locus CHD (chromo - helicase - DNA
binding protein) makes a definitive gender-identification
test possible (Fridolfsoon & Ellegren 1999. J Avian Biol 30,116).
CHD
is present as a pair of duplicated gene loci on the W and Z
chromosomes (CHD-W and CHD-Z, respectively)
[below,
middle].
CHD-W
and CHD-Z
have conserved exons (black boxes), but differ in their
intron lengths (450 vs
600bp,
respectively). A WZ female
is therefore expected to have introns of two different sizes,
whereas a ZZ
male will have only one size
class.