Trump administration would like to pretend some LGBT+ people don't exist, let's look at the facts

There are even more ways in which people identify their gender identity or sexual orientation than the ones mentioned above and as best as our science goes this is not a conscious choice^[8,9] (and even if it was, it wouldn't be anybody else’s business, would it?) but something determined largely or completely by your biology, something that you feel deep inside. If, for example, you are a straight male who feels at home in a male body and feels attraction to females, then you know how deep those feelings are and how inherent to who you are. In part 2 we will look at some aspects of the neurobiology of sex and gender, which should help you find an indication of where these feelings come from.

Describing feelings can be difficult so it’s no wonder different people come up with different labels for who they are and why the terms you may be familiar with do not apply to them. Furthermore, we are talking about the intersection between physical sex, gender identity (and expression) and sexual orientation which can all be non-binary and also move independently from each other at least to some extent^[4]. This creates a rather big canvas of possibilities, which, again, explains why the traditional male and female labels do not apply to any number of individuals you may meet in the real world. So maybe next time you should stop to think before you start talking about “attack helicopters.”

You probably know the human genome is organized into 23 pairs of chromosomes, one set of 23 is inherited from the mother and the other from the father. The “sex chromosomes" make one such pair and are so named, in an undoubted bout of originality, because they are essential in determining the sex of the individual. The chromosomes in the other 22 pairs are called “autosomes” and they also play a role in sexual development. Some people love to point out that “XX=female and XY=male and that’s it!” but the reality is more complicated. Our brain loves to put things into neat categories, yet nature is far too complicated for neat categories.

With that in mind, we can focus on the Y chromosome, which carries a master gene for sex determination known as “SRY.” Initially, XX and XY embryos don’t really seem to differ, it’s only after the SRY gene is activated that those who carry it start developing testicles (see Figure 1). You could say that the “default” developmental pathway is that of developing ovaries.

As the testis develop, they start producing and secreting sex hormones (primarily androgens like testosterone, although estrogens are also produced and needed for normal sexual function and development), which go on to further masculinize the body. Most of the sexual structures have a common point of origin between the two sexes. For example, the primordial gonads can become testis or ovaries while the clitoris and the glans derive from the same precursor, as do the scrotum and outer labia (see Figure 2 below).

^{Figure 2. Representative illustrations of human foetal external genitalia from a sexually undifferentiated stage (upper left) to different stages of differentiation. Adapted from public domain source by Gray's Anatomy of the Human Body.}

But nature doesn’t do categories:

• Sometimes during gametogenesis (the process that gives rise to sperm or egg cells, see Figure 3 to the right) the SRY gene can be copied into an X chromosome and/or be excised from the Y chromosome meaning some people can be XX SRY-positive^[10] or XY SRY-negative^[11]
• Other times, different numbers of sex chromosomes can hitch a ride into a gamete leading to people who can be XXX, XXY or XXXY among other clinically described possibilities^[12]
• Yet other times, people may carry XY SRY-positive sex chromosomes plus androgen receptors that respond differently from what’s typical^[13].

This is an incomplete list but we already have numerous examples of people who behave and look typically “male” or “female” but do not conform to the “XX=female and XY=male and that’s it!” narrative. In other cases, we're talking about different kinds of intersex people who can have genitals/gonads that don’t conform to traditional “male” or “female” categorization. There can even be variability within a particular group, both in terms of physical sexual characteristics and personal identification. For example, XX intersex individuals can present different physical sexual attributes and may choose treatment to further their primary and secondary sexual characteristics into the typical male or female form^[14]. This can happen because, as we review in part 2, the degree of masculinization/feminization of the sex organs can differ from the degree of masculinization/feminization of the brain^[15].