HOW DO GENOMES OF MEN AND WOMEN DIFFER?
We all have 22 pairs of regular chromosomes, called autosomes, and two special chromosomes called sex chromosomes. Women have two copies of a sex chromosome known as the X chromosome. Men have one X chromosome plus a sex chromosome known as the Y chromosome. The Y chromosome is the smallest of all the chromosomes and contains a limited number of genes, many of which are involved in male sexual development.
Many diseases that only affect men are related to variants in genes on the X chromosome. Because males only have one X chromosome, a variant that causes dysfunction or loss of function of a gene on the X chromosome is more easily manifested in a male than in a female. For example, a common form of red–green color blindness that is over 15 times more common in males than females is due to variants (mutations) in two photopigment genes on the X chromosome. Females have two X chromosomes and thus greater probability of having at least one functional copy of the photopigment genes. Other X-linked diseases that are manifested primarily in males include hemophilia and certain forms of muscular dystrophy.
Interestingly, although females carry two X chromosomes, in any given cell one of those X chromosomes is inactivated, and, as a result, gene expression on that chromosome is greatly reduced. X inactivation is believed to occur to prevent the accumulation of excess, potentially toxic levels of gene products of the X chromosome. As a result of X inactivation, females are genetic mosaics, a term used to describe populations of genetically different cells in the same person. A good representation of female genetic mosaicism related to X inactivation is the mottled fur color of tortoiseshell cats. The mottled pattern reflects the expression of different X-linked fur- color gene products on the two different X chromosomes. Depending on which X chromosome is active, either the orange or black pigment is produced.