Three important requirements?
When ‘sex chromosomes’ were discovered and it was assumed that ‘all’ women have XX chromosomes and ‘all’ men have XY chromosomes, it was quickly concluded that the Y chromosome is responsible for masculinization. A little too quickly, actually. Later it was discovered that the SRY gene plays a role. SRY stands for Sex determining Region of Y. This is a very small piece of the Y chromosome – about 1/65,000th of the complete Y chromosome. So it’s the Y chromosome after all that is responsible for determining the sex? No, this is not true either. It was then discovered that a whole bunch of other genes also play a role. Some of those genes are part of the X chromosome, but there are also genes on the 22 other chromosome pairs that can influence the development of a person’s sex characteristics.
Schematic overview for the conversion of cholesterol to other hormones.
A gene’s function may be different than expected if a number of base pairs (the AT and CG combinations that make up a DNA strand) are missing, unexpectedly contain different information, or are actually duplicated.
2 But even if all these genes are laid out exactly by the book, they only function if the right hormones are made. The relationship between genes and hormones is like that between locks and keys: you need the right lock and the right key to make a gene ‘work’.
Although testosterone and estrogen are still commonly called ‘the male hormone’ and ‘the female hormone’ respectively by many people, the reality is far more complicated. This is already evident from the fact that both men and women produce both hormones, albeit in different proportions. These hormones are made from cholesterol via a set of intermediate steps using proteins. If one of the required proteins is missing, the process is interrupted: a hormone is not made, or a lesser amount is made, and the gene that is dependent on that hormone won’t work as expected either.
Bust of Aristotle. Marble, Roman copy after a Greek original of bronze by Lysippos from 330 BC. The mantle of alabaster is a modern addition. Photo: Jastrow 2006.
In addition to genes that are put together differently than expected and hormones that are not or insufficiently produced, it can also happen that a chromosome disappears completely or that an extra chromosome is added. When we’re speaking of examples of this for specific forms of sex diversity, the medical diagnoses Turner syndrome and Klinefelter syndrome show this. With Turner an X or a Y chromosome literally disappears shortly after fertilization, and with Klinefelter each cell has two or more X chromosomes in addition to a Y chromosome.