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.