Y chromosome

In the human genome, the Y chromosome is one of the 23 chromosomes. It is an acrocentric chromosome, found within group G of the human karyotype (the only chromosome within this group that lacks a satellite). The Y chromosome is one of the two sex-determining chromosomes.

The Y chromosome is approximately 58 Mbp (million base pairs) in length, comprising ~1.5-2% of the haploid genome
The Y chromosome has approximately 70-200 genes

In humans and all other mammals, the Y chromosome is found only in males; thus the majority of its genes code for male development. The SRY gene is responsible for the cascade of events which cause a foetus to develop as male. Some of its genes, notably those found in the pseudoautosomal regions, are also found on the X chromosome.

Because the Y-chromosome is transmitted only down the paternal line, it is often used in genealogy to trace family lines. The maternal equivalent is found by analysing mitochondrial inheritance.

Most mammals have one pair of sex chromosomes in each cell. Males have one Y chromosome and one X chromosome, while females have two X chromosomes. In mammals, the Y chromosome contains a gene, SRY, which triggers embryonic development as a male. The Y chromosomes of humans and other mammals also contain other genes needed for normal sperm production.

There are exceptions, however. For example, the platypus relies on an XY sex-determination system based on five pairs of chromosomes. Platypus sex chromosomes in fact appear to bear a much stronger homology (similarity) with the avian Z chromosome, and the SRY gene that is so central to sex-determination in most other mammals is apparently not involved in platypus sex-determination. Among humans, some men have two Xs and a Y ("XXY", or Kleinfelter's Syndrome), or one X and two Ys (or XYY syndrome), and some women have three X chromosomes or a single X instead of a double X ("X0", or Turner Syndrome). There are other exceptions in which SRY is damaged (leading to an XY female), or copied to the X (leading to an XX male).

The X and Y chromosomes are thought to have evolved from a pair of identical autosomes, when an ancestral mammal developed an allelic variation, a so-called 'sex locus' - simply possessing this allele caused the organism to be male. The chromosome with this allele became the Y chromosome, while the other member of the pair became the X chromosome. Over time, genes which were beneficial for males and harmful to (or had no effect on) females either developed on the Y chromosome, or were acquired through the process of translocation.

Until recently, the X and Y chromosomes were thought to have diverged around 300 million years ago. However recent research, particularly that stemming from the sequencing of the platypus genome, has suggested that the XY sex-determination system wouldn't have been present more than 166 million years ago, at the split of the monotremes from other mammals. This reestimation of the age of the therian XY system is based on the finding that sequences that are on the X chromosomes of marsupials and eutherian mammals are present on the autosomes of platypus and birds. The older estimate was based on erroneous reports that the platypus X chromosomes contained these sequences.

Recombination between the X and Y chromosomes could be harmful - it could result in males without necessary genes formerly found on the Y chromosome, and females with unnecessary or even harmful genes previously only found on the Y chromosome. As a result, genes beneficial to males accumulated near the sex-determining genes, and recombination in this region was suppressed in order to preserve this male specific region. Over time, the Y chromosome changed in such a way as to inhibit the areas around the sex determining genes from recombining at all with the X chromosome. As a result of this process 95% of the human Y chromosome is unable to recombine.

Significant genes (and some related genetic diseases) on the Y chromosome include: