A Z chromosome is a
sex chromosome found in all
birds and certain other
animals where the
female is the
heterogametic sex. It is part of the WZ set of sex chromosomes, which are so designated to prevent
confusion with the
XY set of
sex chromosomes for species (such as humans) where the
male is the
heterogametic sex. In the WZ system, the male has two Z chromosomes and the female has only one Z (and one W chromosome).
From the science dictionary at http://biotech.icmb.utexas.edu/