Drives me really nuts. There are all these people who ask these questions that include the terms biological female or male as if it were a real thing. I thought I’d get it off my mind along with some information about genes and get to sleep. As an added plus I can cut and paste this next time!
A person has 22 pairs of chromosomes in their karyotype plus 2 sex chromosomes normally. In abnormal cases there can be a single X chromosome in the sex chromosomes or a number of extra X and Y chromosomes, all of which lead to disorders of development.
During gestation, which is to say the development of a fertilized human ova to a baby, in week eight the sex chromosomes have essentially finished their job. You could remove them except that they are needed for reproduction. Their job is to produce either a pair of ovaries (XX) or testes (XY) which then start producing estrogen or testosterone.
From week 8 on the hormones start causing the developing baby to differentiate the proto genitalia into either the female or male primary sex characteristics. They don’t, at that time have much effect on the rest of the body until about week 15 (going on memory) when the brain differentiates.
Until the child enters puberty little girls and boys are, aside from social and societal development, physically alike. But once they enter puberty the sex hormones cause the primary sex organs to complete their development and secondary characteristics such as a deeper voice for males and breasts for females to develop.
What’s important to note is that all of this happens due to the gonads releasing hormones. It is not the transcription of DNA at puberty. In fact, cross sex hormone therapy wouldn’t work if the cellular make up of men and women were not fundamentally the same. Even the primary sex organs such as the glans penis and the clitoris are fundamentally the same tissue type, just formed somewhat differently.
When a transwoman grows breasts under the influence of estrogen they are actual real breasts like any woman has. When her skin grows softer it softens for the same reasons that other women have soft skin.
Even when it comes to vaginoplasty, some care is taken to use, wherever possible, the same tissues as would be present in female genitalia when reconstructing from the original male genitalia. The big unfortunate exception is the vagina, although research at Wake Forest University shows a path for that to eventually be lab grown.
The last refuge of the bio sex argument is sometimes the brain, but there have been many, many articles now in peer reviewed journals about some sexually dimorphic areas of the brain where transwomen are, on average, closer to ciswomen than cismen and transmen are, on average, closer to cismen than ciswomen. There’s also at least one study showing similar results for a connectivity map.
Although each study has relatively modest P (probability of being true) values, the combination of so many studies taken together would make it highly unlikely that this is merely random chance.
I’m intentionally ducking the issue of causes of being transgender, but I will note that the results of identical twin studies indicate that it clearly is, to some extent, heritable.
My Gifted Life 