The W chromosome
sex chromosome is found in all
birds and certain other
animals where the
female is the
heterogametic sex. The W
chromosome causes the individual to become
female in such species. It is part of the WZ set of sex chromosomes, where the female has one W chromosome and one
Z chromosome, and the
male has two Z chromosomes.
The information in this writeup was taken from the science dictionary at http://biotech.icmb.utexas.edu/; I oversaw the development of the dictionary (the website was mothballed in 1998) and I believe I wrote the entry this is based on.