We all have at least one X chromosome. Some of us have two. We know that two X chromosomes makes a female, and one X plus a Y makes a male. But there are many more amazing facts about the X chromosome and its genes!
10 Only one X is active
Men have only one X, and to correct the imbalance between the genders, females have only one active X in each cell. The other X is inactive and is called a Barr body (see arrow pointing to Barr body in illustration below). In humans, the inactivation of one of the X chromosomes begins not long after fertilization, when the embryo is a curved double layer of cells. A gene called Xist is triggered on one of the Xs, and coats the gene in Xist RNA. From that point on, every time the cell divides, the new cell will have the same inactive chromosome as the original cell. The only time this does not occur is during the formation of eggs in the ovary (LINK 1).
A striking example of X inactivation is the calico cat. The X of a cat has a coat color gene that is either orange or black. Calico and tortoiseshell cats have one X with the orange gene and one with the black gene. Because some cells have an active orange gene, while others have an active black gene, the cat is a mixture of black and orange, plus white (on a different gene) if she is a calico. Most calico and tortie cats are female. The only male cats with this coat pattern are abnormal because they have two Xs as well as a Y. They are anatomically male, but sterile. Like females, one of their X chromosomes becomes inactivated and they will have both orange and black on their coats (LINK 2).
9 X inactivation is not always random
In mice and marsupials, the X that came from the father (the paternal X) is always the inactive one. In humans, the process is basically random. However, there are factors that can cause the inactivation to be skewed — that is, either the maternal or the paternal X is favored for inactivation (LINK 1).
8 X-linked disorders occur mostly in males
Some diseases, such as hemophilia, are located in genes on the X chromosome. The gene is recessive, meaning that an X without the gene can overcome it. But males have only one X. If the X has the hemophilia gene, there is no corresponding gene to counteract it. Problems like this are called X-linked disorders. In most cases, the female is a carrier and does not have the disorder herself. According to traditional genetics, she would need to have the disordered gene on both Xs (LINK 3).
7 But Females CAN have recessive x-linked disorders, too!