Understanding the X-Chromosome

Sex chromosomes determine the sex of most animals, including mammals. There are two types of sex chromosome, namely the X-chromosome and the Y-chromosome. Every cell contains a pair of sex chromosomes - of 23 chromosome pairs in human cells, only one pair is that of the sex chromosomes. The sex chromosome pair may be either XX or XY, and all cells in an organism contain the same type of sex chromosome pair.

So, for example, if a cell contains XX chromosomes, all the cells in that organism will have XX chromosomes and the organism will be female. And if a cell contains XY chromosomes, all the cells in that organism will have XY chromosomes and the organism will be male.

One X chromosome in both males and females is inherited from the mother. Females get the second X chromosome from the father and males get their Y chromosome from the father.

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.

Many genetic disorders are related to genetic defects or genetic changes of the X-chromosome genes. Males are more likely than females to have X-chromosome related disorders.

Some X-chromosome related disorders are -

• Jacobs syndrome
• Turner syndrome
• Klinefelter's syndrome
• Triple X syndrome
• XX male syndrome