The Y chromosome is responsible for determining male gender in mammals. The Y chromosome contains the SRY gene, which is necessary for the development of the testes in males. Without this gene, no testes would develop and a fetus would grow into a female. The Y chromosome is extremely quick to evolve, with the human Y chromosome differing from that of a chimpanzee by 30%.
In many ectothermic vertebrates, there are no sex chromosomes and in those who do have sex differences, gender is not determined by sex chromosomes but by the environment. In reptiles, for example, gender is selected according to the incubation temperature.
The two sex chromosomes in humans are thought to have evolved from a pair of autosomes due to the presence of an allelic variation on one of the chromosomes in an ancestral mammal. The chromosome with the alteration became known of as the Y chromosome and the remaining autosome became the X chromosome. The Y chromosome slowly developed or acquired genes that were advantageous to males but not to females.
The Y chromosome spans approximately 58 million base pairs, contains 86 genes, and represents around 2% of the total DNA in a human male. Traits that are passed from father to son on the Y chromosome are referred to as holandric traits, meaning they only occur in males. Aside from very small regions present at the telomeres, the Y chromosome is unable to recombine with an X chromosome. This majority portion of the Y chromosome is referred to as the non-combining region of the Y chromosome and the single nucleotide polymorphisms in this region are used to determine paternal ancestry.
Recombination of the X and Y chromosome results in males losing necessary genes from the Y chromosome and females acquiring unnecessary or even harmful genes on the X chromosomes. On the Y chromosome, this led to genes that benefit males only gathering near the genes that determine gender. It also led to suppression of recombination in this area, to preserve the male-specific genes. Eventually, alterations in the Y chromosome prevented recombination of these areas altogether and 95% of the Y chromosome is now unable to recombine with the X chromosome. The tips of the two sex chromosomes do recombine but the rest of the Ychromosome is passed on to offspring unaltered.