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GLOSSARY

X chromosome

The X-chromosome is one of the two sex chromosomes, used to determine genotypical sex. One X chromosome comes from your mother's egg; sperm may carry an X or Y chromosome. Typically, females have two X (XX) chromosomes, while males usually have an XY pairing. Some people may have atypical chromosome combinations, such as XXY, also known as Klinefelter syndrome.

What is the X chromosome?

The X chromosome is one of the two sex chromosomes found in humans and most mammals (the other being the Y chromosome). Females have two X chromosomes in their cells, while males have X and Y chromosomes in theirs. The X chromosome is around three times larger than the Y chromosome and contains approximately 900 genes. The Y chromosome contains about 55 genes in comparison. 

How the X chromosome impacts male fertility

Klinefelter syndrome is a genetic condition in which a male is born with an extra copy of the X chromosome (so, XXY). It may adversely affect testicular growth, resulting in smaller than normal testicles, which can lead to lower production of testosterone and negatively impact fertility. Though most individuals with Klinefelter syndrome produce little or no sperm, assisted reproductive procedures can help. 

There is some evidence that specific gene mutations on the X chromosome can also contribute to azoospermia, in which there is zero sperm in the ejaculate, and male factor infertility.

History of the X chromosome

The naming of the X chromosome happened in the late 1800s and is credited to the German biologist Hermann Henking. Hanking noticed while studying wasp sperm cell mitosis that some sperm cells had 12 chromosomes, while others had 11. He called the 12th chromosome the “X element,” presumably because it was unknown. A few years later at the University of Kansas, Clarence Erwin McClung found that half of grasshopper sperm contained a chromosome similar to Henking’s X, and confirmed that the X and Y chromosomes determined sex. 

Sex hasn’t always been determined by chromosomes. In many reptiles, the temperature at which their eggs are incubated is the main determinant of the sex of their offspring. However, warm-blooded mammals with internal reproduction don’t have the same variability to their environment. As mammals evolved approximately 300 million years ago, they evolved what is now known as the X and Y chromosomes.

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