The Y chromosome is one of the 2 sex-determining chromosomes in most mammals, including humans. In mammals, it contains the gene SRY, which triggers testis development if present. The human Y chromosome is composed of about 50 million base pairs. DNA in the Y chromosome is passed from father to son, and Y-DNA analysis may thus be used in genealogy research. With a 30% difference between humans and chimpanzees, the Y chromosome is one of the fastest evolving parts of the human genome.