Some Features of the X-Chromosome
The X-chromosome contains around 155 million base pairs representing some 2000 genes. This is more than the genes contained in the Y-chromosome - around 78. The entire human genome has around 20,000 to 25 000.
Some 1,098 of the X-chromosome genes are capable of protein-coding. These genes are shorter in length than other genes in the body. Some X-chromosome genes - about ten percent of them - belong to the cancer-testis (CT) antigens family. These genes are found in the testis and also in some forms of cancer.
At an early stage in embryonic development, X-inactivation takes place. This process, also known as Lyonization, involves the random inactivation or switching-off of one of the two X-chromosomes in females. The process operates to ensure that a female does not have twice as many gene products as a male, with his one X-chromosome. In a sense it stops a female's cells from becoming overloaded. An umbrella term for this genetic regulatory mechanism is dosage compensation, which ensures that the phenotypic express of traits that originate from X-chromosome gene products are the same in males and females.
X-inactivation occurs only in somatic cells, not in the egg and sperm cells. In any given cell one X- chromosome will be active, and the other inactive.
The X-chromosome - and the Y-chromosome too - has two pseudoautosomal regions, PAR1 and PAR2, which contain nucleotide sequences. The genes in these regions are not inactivated. So everyone has a functional pair of these genes. These genes are necessary for healthy development.