In 1890, the X and Y chromosomes were discovered. It was found that the men who were tested had 46 chromosomes, including an X and a Y, while women who were tested also had 46 chromosomes, including 2 X chromosomes.
So obviously the conclusion was that the Y chromosome defined masculinity. A reasonable conclusion.
Fast forward 50 years... and it was found that some men had 47 chromosomes, including 2 X's and a Y, while some women had 45, including only one X. Still no problem with the "Y chromosome defines masculinity" idea.
Then... it was found that fully 1 in 300 men weren't 46,XY. Some women were.
Oops.
After DNA was discovered in the 50s, it was found that the SrY gene, usually found on the Y chromosome, sometimes was missing. And sometimes had been translocated to another chromosome, hence 46,XX men and 46,XY women. So SrY defined masculinity.
Then.. it was found out that some men didn’t have an SrY chromosome, not anywhere. Some women did. Other genes were involved. Worse, other factors, such as Androgen Insensitivity made 46,XY people female, and Congenital Adrenal Hyperplasia masculinised 46,XX people.
Then in the 70s, other syndromes, such as 5alpha-reductase-2 deficiency were identified, which caused babies to look like one sex at birth, then the other at puberty. Worse, in some places 1 in 50 infants had this natural sex change, it was not rare there.
Science 1974 Dec 27; 186 (4170): 1213-5
In an isolated village of the southwestern Dominican Republic, 2% of the live births were in the 1970's, guevedoces....These children appeared to be girls at birth, but at puberty these 'girls' sprout muscles, testes, and a penis. For the rest of their lives they are men in nearly all respects.
In the 90s, it was found that hormonal hiccups in the womb caused some parts of the body to develop as one sex, others as the other, regardless of genetics.
Male–to–female transsexuals have female neuron numbers in a limbic nucleus. Kruiver et al J Clin Endocrinol Metab (2000) 85:2034–2041
The present findings of somatostatin neuronal sex differences in the BSTc and its sex reversal in the transsexual brain clearly support the paradigm that in transsexuals sexual differentiation of the brain and genitals may go into opposite directions.
It's a matter of timing during foetal development. Sometimes a boy is born looking like a girl, sometimes a girl is born looking like a boy, regardless of chromosomes.
This is complex stuff. We don't teach the Theory of General Relativity in grade school, Newtonian physics or at most Special Relativity (far simpler) is enough. Similary, "XX is female, XY is male" is good enough unless you do medicine or biology in college.
It's only really relevant when talking about Trans or Intersex people, just as Relativistic effects only become relevant in the domain of the very big, very small, or very very fast, close to 186,000 miles a second.
People do *not* need psychiatric help when they think that things get heavier, more massive, as they go faster... while lengths contract. People do *not* need help when they think their sex is something different from their genetics.
Intersex people exist. Trans people exist. They are unusual, so trying to apply the usual approximations is as silly as trying to apply Newtonian physics to things moving close to or at light speed. Legislating such things is as insane as legally ruling that Pi=3... as has been done in the past.
I hope this helps explain.
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