Let's Talk About X, Baby!

Have you ever heard the phrase "less is more?"  That quote applies to genetics, too!  Every female is born with two copies of the X chromosome, while males are born with one copy of the X chromosome and one copy of the Y chromosome. While this difference accounts for the designation of an individual's assigned sex, it also creates a large genetic problem called aneuploidy.  A female individual now has two copies of the same chromosome, which means she has too many genes.  Extra chromosomes can cause birth defects, like Down Syndrome which occurs after an extra copy of the 21st chromosome is made.

Karyotype Displaying Extra 21st Chromosome

So, how does one deal with this issue? The answer is X-inactivation, in which one copy of the X chromosome is condensed into a Barr body that becomes inactive because genes are not transcribed from it.  This occurs in autosomal cells during early embryonic development. This process is known as lyonization, named after Mary Lyon, the British scientist who discovered it.

Picture of Mary Lyon

To me, the most interesting part of this process is that it's completely random! It's impossible to know which chromosome will be deactivated, and in all female individuals, the same chromosome isn't deactivated everywhere.  The only exception to this is in kangaroos where the paternal X chromosome is activated over the maternal X chromosome.

A Kangaroo

This random activation and inactivation of X chromosomes means that all over a female individual's body, different genes are being expressed. This is called mosaicism, where two or more cell populations that have different genotypes are expressed in a single individual.  The best example of mosaicism caused by X-inactivation is found in a calico cat.  All calico cats are female because of X-inactivation.  For a male cat to be calico, they would have to have an "X"tra somatic cell.

A Calico Cat Displaying Mosaicism

This cat had parents that were likely orange and black.  On one X chromosome, there was a gene coding for black fur and on the other chromes there was a gene coding for orange fur.  Since X inactivation is random, the cat has multiple patches of black and orange fur across her body.  And if you were wondering, yes, female women are also mosaics!  Although mosaicism doesn't come through as obviously in female humans, it shows up in more subtle ways like a lack of sweat glands in certain parts of a woman's body.

In conclusion, ladies don't let anyone tell you otherwise; you are literally a mosaic and a work of art.