<Tr> <Th> RefSeq </Th> <Td> NC_000024 (FASTA) </Td> </Tr> <Tr> <Th> GenBank </Th> <Td> CM000686 (FASTA) </Td> </Tr> <P> The Y chromosome is one of two sex chromosomes (allosomes) in mammals, including humans, and many other animals . The other is the X chromosome . Y is the sex - determining chromosome in many species, since it is the presence or absence of Y that determines the male or female sex of offspring produced in sexual reproduction . In mammals, the Y chromosome contains the gene SRY, which triggers testis development . The DNA in the human Y chromosome is composed of about 59 million base pairs . The Y chromosome is passed only from father to son . With a 30% difference between humans and chimpanzees, the Y chromosome is one of the fastest - evolving parts of the human genome . To date, over 200 Y - linked genes have been identified . All Y - linked genes are expressed and (apart from duplicated genes) hemizygous (present on only one chromosome) except in the cases of aneuploidy such as XYY syndrome or XXYY syndrome . (See Y linkage .) </P> <P> The Y chromosome was identified as a sex - determining chromosome by Nettie Stevens at Bryn Mawr College in 1905 during a study of the mealworm Tenebrio molitor . Edmund Beecher Wilson independently discovered the same mechanisms the same year . Stevens proposed that chromosomes always existed in pairs and that the Y chromosome was the pair of the X chromosome discovered in 1890 by Hermann Henking . She realized that the previous idea of Clarence Erwin McClung, that the X chromosome determines sex, was wrong and that sex determination is, in fact, due to the presence or absence of the Y chromosome . Stevens named the chromosome "Y" simply to follow on from Henking's "X" alphabetically . </P>

A gene on the y chromosome that determines maleness is