The X chromosome is one of two sex chromosomes. Humans and most mammals have two sex chromosomes, the X and Y. Females have two X chromosomes in their cells, while males have X and Y chromosomes in their cells. Egg cells all contain an X chromosome, while sperm cells contain an X or a Y chromosome. This arrangement means that during fertilization, it is the male that determines the sex of the offspring.


So this, because I'm a female, is truly one of my favorite chromosomes. As you know, females have two X chromosomes. They're quite large in comparison to the male chromosomes. They are carried by the egg, and so consequently you pass on--if you have an egg--you can only pass on an X chromosome to your offspring. What's also quite interesting is the number of genes that are found on the X chromosome. It is predicted that there are approximately 155 million base pairs, which translates to about 900 to 1,400 genes on the X chromosome. Meaning that it carries about five percent of the total DNA in the entire cell. Which is quite in contrast to the Y chromosome, which is considerably smaller. Again, if you look at the X chromosomes in the genes that it carries, often times you'll see that sex-linked disorders are carried on the X chromosome. Which is why, as I stated before, they're more predominant in male, because there's not a protective mechanism against having a mutation on one of those genes because we don't have the normal copy of that gene on the X chromosome.

- Carla Easter, Ph.D.