Only room for one: X Inactivation
Chromosomes are tightly bound wired DNA. Every cell has autosomal and sex chromosomes. The sex chromosomes in the case of humans are XX for females and XY for males. However, in females only one of the X chromosomes is active, the other is not. This phenomenon is called X chromosome inactivation. Why does this inactivation happen and how does it happen?
Why is one of the X chromosomes inactivated?
The Y chromosome in the case of humans, is small and carries very few genes as compared to the X chromosome. To maintain the balance of the genes between both genotypes, one of the X chromosomes is inactivated. In the case of some other animals, like fruit flies, the X chromosome is not inactivated, whereas male fruit flies show hyper-activated X chromosomes.
When does X inactivation take place?
The genetic material for a zygote is provided by both male and female counterparts. In the case of a female child, one of the X chromosomes comes from the mother and the other comes from the father. One of out of these two chromosomes is inactivated and it is completely random. Hence, any of the two can be inactive. The inactivation occurs in the embryonic stage. At the eight-celled stage of the embryo, one of the X chromosomes is inactivated and loses its potential for expression.
How does X inactivation occur?
The X chromosome has a center in it called the X inactivation center which has control elements like Xist. When this Xist is blocked by a blocking factor, the chromosome is activated. The cells produce a limited amount of this factor so that only one chromosome can be blocked and in turn, be active. The blocking protein plays an important role because, just before the Xist, there is a region in the chromosome that produces a non-protein coding unstable RNA.
If there is no blocking factor, this RNA gets stable in nature and is produced in high amounts. This RNA accumulates around the chromosome and modifies the packaging of the chromosome, i.e. it recruits the proteins that modify the packaging of DNA. These modifications make the DNA so dense that it turns into a structure called the Barr body. Hence, it cannot take part in any biological function.
In the case of intersex genotypes like (XXX), only one of the X chromosomes is activated because the blocking factor is produced in very little amount that only one X chromosome is allowed to be active.
So, if you are a female, there are cells in your body that express your mother's X chromosome and some cells in your body that express your father's X chromosome!
References:
Lyon, M. (2007). X-chromosome inactivation and human genetic disease. Acta Paediatrica, 91, pp.107–112. doi:https://doi.org/10.1111/j.1651-2227.2002.tb03120.x.