A Y chromosome is one of the sex chromosomes in humans and other mammals, with genetic code determining whether an organism will be genetically male or female. The presence of a Y chromosome will usually cause someone to be male, except in the case of certain genetic disorders. The complementary chromosome is the X chromosome; women generally have two X chromosomes in their karyotype, while men inherit an X and a Y.
Not all organisms have sex chromosomes, and those that do may have a complicated system of inheritance when it comes to sex. This article focuses specifically on the role of the Y chromosome in humans. It is believed that initially, ancestors of humans inherited two basically identical sex chromosomes, and over time they diverged into an X and Y chromosome. The Y chromosome is somewhat shorter than the X chromosome, and primarily contains genetic material related to male sex characteristics.
One of the most important genes among the 58 million base pairs on the Y chromosome is the SRY gene, which determines the formation of testes. When the testes form, a cascading series of reactions happens as they produce sex hormones like testosterone, contributing to the development of secondary sex characteristics like chest hair and a deep voice. The genetic material in the this chromosome makes up about 2% of the genetic material in any given cell, a relatively small amount, and historical evidence suggests it was once larger and is actually shrinking with time.
Genetic disorders involving the this chromosome usually cause problems with fertility or the development of sex characteristics. In some cases, people inherit an extra X chromosome, ending up with an XXY karyotype and a condition called Klinefelter's syndrome. The subject is male, but may have fertility problems and other issues.
Men pass their Y chromosomes on to their sons, and if they have genetic disorders on the Y chromosome, their sons will have these disorders as well. In contrast, if they are carriers for genetic disorders seen on the X chromosome and they have daughters, their daughters will also become carriers and may be at risk of developing the disorder if it is dominant or if they inherit another copy of the defective gene from their mothers. One example of a sex-linked genetic disorder is hemophilia, a condition primarily seen in men who inherit a single defective X chromosome and lack a corresponding allele on the Y chromosome to inhibit the expression of the deleterious gene.