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X is for X Chromosome Inactivation

Let’s take a journey back to high school biology class! As you know, the X chromosome is part of the pair of sex chromosomes. Women have two and men have one. It contains roughly 5% of your cells’ total DNA. The other 22 pairs of autosomes code for non-sex-linked characteristics. Each chromosome codes for the same type of gene, though which variation it codes for may differ. As you can imagine, abnormalities would arise if the genes on both chromosomes were expressed. So, your body turns off, or inactivates, one copy very early in embryonic development. Some of your cells will express the genes on the chromosome inherited from your mother, and others the genes from your father. The inactivation of the second chromosome is known as X Chromosome Inactivation.

X Chromosome Inactivation

The inactive chromosomes’ genes are effectively “silenced” by being packaged into a heterochromatin. This prevents the genes from being transcribed into the corresponding protein. The inactivation process occurs at random and does not reverse. It is responsible for the heterozygous (or “carrier”) form of certain diseases. One example is if a person has Sickle Cell Trait. Not all their marrow cells code for the production of a defective red blood cell. Therefore this person is said to have sickle cell trait, not sickle cell disease. While this is a gross oversimplification of sickle cell disease, it illustrates the point.

X chromosome inactivation occurs any time there is more than one copy of a gene, so it occurs in all the autosomal pairs of chromosomes. With the XY chromosomes of males,  the genes on the Y chromosome are not inactivated because they are not also contained on the X chromosome.

Identical Twins and X Chromosome Inactivation

X chromosome inactivation is the reason why you will notice subtle differences between identical twins. These individuals possess all the same genetic material, however the expression of that genetic material may not be the same for all genes. Since the inactivation of chromosomes is random, one twin may have the maternal copy of a particular gene expressed and the other twin the paternal copy. How crazy is that?! The mechanism by which this occurs is not entirely known at this point, but some believe it has to do with RNA. If you are interested in reading more on this topic, I linked a study below.

 

 

The statements contained in this article are for informational purposes only. They are not meant to take the place of consultation with a medical professional, nor are they meant to diagnose, treat, cure, or prevent any disease.

 

Additional Reading:

https://www.ncbi.nlm.nih.gov/pubmed/12429693