The X and Y chromosomes, also known as sex chromosomes, differ greatly from each other. But in two regions, they are practically identical. We’re interested in studying how the process of evolution shaped the X and the Y chromosome in gene content and expression and how that subsequently affects literally everything else that comes with being a human. In humans, each cell contains 23 pairs of chromosomes, for a total of 46. The first 22 pairs look the same in both males and females. The last pair, the sex chromosomes, differ.
Typically, females have two X chromosomes, while males have one X and one Y chromosome. I say typically because I don’t know which one of us is normal and I don’t like saying something is normal. Variations in sex chromosomes can be pretty typical.
Historically, most clinical research was conducted in men, we’re now finding that treatments designed for men don’t work so well in women. There is, however, a massive sex difference. The sex chromosomes have been evolving over the past 200 million years, she explained. At first, the chromosomes were identical. Over time, the Y chromosome started to break and rearrange itself. These inversions made it more difficult to recombine with the X chromosome. Now, the X chromosome has about 1,100 genes while the Y chromosome has just 27 unique genes.
And yet, there are two regions called pseudoautosomal regions (PAR1 and PAR2) on the tips of the sex chromosomes that can pair with each other and swap DNA. This is called recombination. Wilson says a sex-determining gene called SRY sits near the boundary of the regions on the Y chromosome. Individuals who inherited the Y chromosome “went on to make testes, and those who didn’t made eggs.” In some cases, SRY can transfer to the X chromosome.
A deficiency in PAR1 recombination has been linked to Klinefelter syndrome, a genetic condition where males are born with an extra X chromosome. This occurs in about 1 out of every 500 births in the U.S. There are other sex-linked differences besides Klinefelter Syndrome. One, called Turner syndrome, happens in about 1 in 2,000 or 2,500 females. Those born with this variation have one missing or structurally altered X chromosome. Another is De la Chapelle syndrome, in which individuals have two X chromosomes, but have a male appearance. Wilson said the field needs to develop new methodologies to analyze the sex chromosomes, recognizing the complications and appreciating the variations.
Source: National Institute of Health