"Let's talk about sex, baby; Let's talk about you and me; Let's talk about all the good things; and the bad things that may be." That's right Salt-n-Pepa, it is time to talk about sex….sex chromosomes, actually. It may not have been what Salt-n-Pepa was talking about but we will talk about the bad things that could be when it comes to sex determination.
Male humans have an X and a Y chromosome, while females have two X chromosomes. The X and Y chromosome actually have few of the same genes, which means that a male has only one copy of genes on the X. Females instead have two copies. In a female one X comes from the father and one X is provided from the mother. The more the merrier? Not exactly.
Therefore, the cell needs to find a way to silence (prevent expression of) one X chromosome in a XX individual. The cells in your body actually do this in random fashion early in development. Therefore, the X chromosomes from dad may be inactivated in one cell, while the X chromosome that comes from mom may be inactivated in another cell. This is called genetic mosaicism.
You can actually visualize genetic mosaics in some animals where the gene for coat color occurs on the X chromosome. If the father and mother of an animal provide genes for different coat colors, then the coat color will be a patchwork. Calico cats are an excellent example. Since the gene is present on the X chromosome, calico cats are predominantly female.
A calico cat has a gene for orange coat color on one X chromosome and genes for black or white coat color on the other X chromosome. Since females inactivate one X chromosome at random, the calico cat's coat will be a mix of the colors. Some cells will express the coat color from one x chromosome, while others will express the coat color from the other x chromosome.
Can you imagine what would happen if the genes for human pigmentation where on the X chromosome? How would it affect the appearance of a female human? A male human?
In some special instances, a person can get more than two X chromosomes. A person who is XXY is male has Klinefelter's syndrome (a condition of having an extra X). Alternatively, a person who has one X is female, a condition called Turner's syndrome. What this fact tells us is that it is the presence of the Y that determines sex in humans. However, it is not that straightforward. Turner's syndrome patients are infertile, and have a number of other health concerns. The same can be said for Klinefelter's syndrome.
The process of X silencing is called X inactivation. X inactivation results in the heterochromatization of the chromosome. If you look at a female's cells under the microscope you can actually see the chromosome that has been inactivated. It is a clump in the nucleus—a structure called a Barr body. This chromosome won't be transcribed.
You may be wondering how this process of inactivation works. You've learned from the regulation theme that there are lots of ways to interfere with the production of a protein. We've told you that X inactivation is transcriptional inactivation. This process occurs through the use of a special RNA, called Xist that actually promotes chromatin changes in the structure of the X chromosome. This makes it take on a more condensed structure that is less accessible for the transcription machinery.
Red Alert. It turns out that x inactivation doesn't always work by silencing one chromosome. If you look throughout different organisms you will see the many different solutions that organisms have arrived at to limit the amount of transcription from the X sex chromosome. Some dial down transcription down by half as much on one chromosome. The way that the cell does this is called dosage compensation.