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.
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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.
Deputy Chief, Education and Community Involvement Branch
Dr. Easter is the education specialist with the Education and Community Involvement Branch of the National Human Genome Research Institute. From 2003 to 2006, she was director of outreach for Washington University School of Medicine's Genome Sequencing Center in St. Louis. Before assuming her position at the NIH, Dr. Easter was a research associate in the Department of Education at Washington University, where she explored the notions of science among secondary students, educators and administrators. She served as project associate for the Quality Education for Minorities Network and the Pre-College Coordinator for the NASA Summer High School Apprenticeship Research Plus program.