A bit of genetics that most of us know about is what makes a boy a boy, and a girl a girl. It’s the X and Y chromosomes. At the very beginning of our existence each of us received one X chromosome from our Mums via the egg, and while the girls received another X chromosome from their dads, via the sperm, the boys got a Y chromosome. The Y chromosome in the cells of a male embryo directs it to develop into a boy, while with two X and no Y chromosome the female embryo develops into a girl. Now, you might notice that there is an imbalance here. We all have two each of all the other chromosomes, but for the sex chromosomes (X and Y) the girls have two Xs while the boys only have one X (and a Y). While the Y chromosome contains few genes, mostly involved in “maleness”, the X chromosome contains quite a few genes involved in important processes such as colour vision, blood clotting and muscle function. In order to even up the “dosage” of X chromosome genes between male and female cells, one entire X chromosome is switched off in female cells. This is called X-chromosome inactivation and happens very early in the womb. In this process cells randomly switch off either the paternal or maternal X chromosome, so that when a girl baby is born her body is a mixture or chimera of cells where either the maternal or paternal X-chromosome is switched off. The way that this happens involves the type of epigenetic tags that we have discussed and it has been known for decades that female cells contain one very compact X chromosome called the Barr body that can be seen under the microscope, and this is the inactive X chromosome.
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