A sex chromosome is a type of chromosome that participates in sex determination. Humans and most other mammals have two sex chromosomes, the X and the Y. Females have two X chromosomes in their cells, while males have both X and a Y chromosomes in their cells. Egg cells all contain an X chromosome, while sperm cells contain an X or Y chromosome. This arrangement means that it is the male that determines the sex of the offspring when fertilization occurs.
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Typically in mammals, the gender of an organism is determined by the sex chromosomes. In the case of humans, this happens to be the X and the Y chromosomes. So as you may recall, if you are XX, you are female. If you are XY, you are male. And in the case of other mammals, this is often similar, although they may have different names for those chromosomes as well. What's really interesting is that you have a large discrepancy in the size of these two chromosomes, with the X chromosome being much larger than the Y chromosome. They also carry distinctly different genes. And what's also very fascinating about this is, I don't think it's really been determined exactly what it is on each of those chromosomes that make someone male or female in particular. It's kind of a combination of many of those genes. In fact, the sex chromosomes are quite interesting, not just because of what they cause or what their role is, but also because they're so different from the other chromosomes that we have in the complete human genome.
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.