Most people know that the X chromosome is one of the two sex chromosomes; inherit an X from each of your parents and you will likely be deemed “biologically female.” Get an X from your mother and a Y from your father and you will likely be deemed “biologically male.” Unlike the other twenty-two pairs of chromosomes, the sex chromosomes are not identical in structure.
As historian Stephen G. Brush writes, the term “X chromosome” was coined by C.E. McClung, for “accessory chromosome.” McClung noticed that in some organisms only one sex had the “X.” Sex determination by chromosome, though, was discovered in 1905 by Nettie Stevens and Edmund Wilson, working independently. The nature of the XY relationship was determined first by Dr. Stevens. Examining mealworms, she noticed that the chromosomes were different in males and females. Females contained twenty large chromosomes, while males contained nineteen large chromosomes and one small chromosome. Stevens deduced that biological sex was determined by this last pair of chromosomes, and that the female-determining feature was larger.
The X chromosome does more than determine sex. We inherit one copy of each chromosome from each parent, and the different versions of genes present on each copy, called alleles, help determine our individual characteristics. Since the X chromosome is larger, most of the genes present on the X are simply not present on the shriveled, puny Y. For this reason, males have only one copy of certain genes. The X chromosome completely calls the shots. Males are therefore particularly susceptible to inherited conditions carried on the X. Colorblindness, various types of hemophilia, and pattern baldness are all more common in males since all they need is one copy of the disease-causing allele. Females who only have one copy of the affected allele can pass the condition on but will not themselves be affected.
The chromosome has other jobs, too. The function of the genes located on the chromosomes is to make proteins. Those proteins do everything in the body from build your liver to determine how you experience onion soup. If females made proteins from two copies of some genes while males only made one copy’s worth, the females would produce way more and the resulting differences might make the sexes biologically incompatible. Females always have two Xs, so expression of X chromosome genes needs to be halved in females. This is known as dosage compensation. In mammals, one copy of the X in each female is shut down into a condensed, inert blob called a Barr Body. Some nematodes turn off half of gene expression in each X; fruit flies double it in males. Marsupials such as kangaroos take a different tac; they selectively shut down the paternally-inherited X chromosome. In other words, for everything to work out physiologically, the X has to get creative.
Editors’ Note: An earlier draft of this post used the word “gender” instead of “sex.” We apologize for the error.