A sex chromosome found in humans, fruit flies, and certain other animals where the male is the heterogametic sex. The Y chromosome causes the individual to become male in most mammal species and carries few other genes besides those dictating sperm development and triggering appropriate hormonal output.
It is part of the XY set of sex chromosomes, where the male has only one X chromosome and usually a Y chromosome (fruit fly males can also have just one X and nothing else), and the female has two X chromosomes.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.