The X and Y of sex
In humans, whether our sex organs develop into testes or ovaries is determined largely by X and Y chromosomes
Males and females have 23 pairs of chromosomes. Of these, 22 pairs (the autosomes) are the same in both sexes, but the last pair – the sex chromosomes – are very different. Females have two X chromosomes, males an X and a Y.
Human X and Y chromosomes evolved from a normal pair of autosomes. At some point in human history, the Y lost the ability to pair up with the X (except at its very ends). As a result, it cannot swap DNA with the X during meiosis (cell division to produce sperm and eggs) and it cannot be repaired.
The tiny Y chromosome has been whittled away, and is now a fraction of the size of its sister X. It has less than 100 genes on it, while the X has about 2,000. Some have even suggested that the Y chromosome will eventually dwindle away to nothing.
For the first few weeks after fertilisation, male and female embryos look identical. A trigger is needed to make an embryo become a male.
That trigger is the SRY gene on the Y chromosome. SRY is responsible for switching on a genetic programme that triggers the formation of the testes, which go on to make the ‘male’ hormone testosterone.
Females typically have two X chromosomes in each cell and form ovaries instead of testes. As the physical changes seen in the developing female are less obvious than in the male, female development was once thought of as the ‘default’ process. However, we now know that approximately the same number of genes are expressed in the developing ovary as in the developing testis, and some of these may actually be working to prevent testis development.
Although females have two X chromosomes, they do not get a ‘double dose’ of the products of the X genes because one X chromosome in each cell is shut down. The inactive X chromosome is known as a Barr body.
The X chromosome to be inactivated is chosen by chance in each cell, so different versions of genes that are located on the X chromosome may be active in different cells. The characteristic coat patterns of tortoiseshell cats, for example, are a result of this chance inactivation. Some genes escape X-inactivation and sometimes X-inactivation can be imbalanced (‘skewed’) in a particular patch or group of cells.Lead image:
Wessex Reg. Genetics Centre/Wellcome Images CC BY NC ND