|MadSci Network: Cell Biology|
The genetic material (called DNA) which makes up genes is organised into groups called chromosomes. Each cell in our body contains 46 chromosomes and they dictate how our bodies grow and what we look like. We inherit half of our chromosomes from our mother and half from our father. These two lots of 23 chromosomes double up, so we actually inherit two full sets of instructions.
Your gender is determined by your chromosomes. The sex chromosomes are called X and Y. If you inherit one X chromosome and one Y chromosome from your parents, you will be a male. If you inherit two X chromosomes you will be a female. Because your mother only has X chromosomes, you will always inherit an X chromosome from your mother, so your gender is actually determined by whether you inherit an X or a Y chromosome from your father. One X chromosome contains enough information to dictate all the necessary growth requirements and so the other X chromosome in females is not needed. This second X chromosome becomes very tightly packed up so the information on it can't be used. This is called a Barr body, named after Murray Barr who discovered it.
This was the first of several questions about Barr bodies, some of which also asked how Barr bodies were used in the olympics. Obviously, if Barr bodies are only found in women, and they are easy to identify, then screening atheletes for Barr bodies is a quick means of determining their genders - so men don't try to compete in women's events and vice versa.
Try the links in the MadSci Library for more information on Cell Biology.