The Y chromosome is a chromosome that decides the sex in most mammals, including humans. It contains the gene SRY, which triggers testis development, so making an animal male or female.
Normal females have two X chromosomes, and normal males have one X and one Y. An egg always carries a single X, while sperms carry either an X or a Y. That is how sex is determined in humans.
Images for kids
Ideogram of human Y chromosome. For band names, see locus.