No such thing as a "gender." It is a made-up word. It means different things to different people, which means it means nothing.

"Gender" was never used a few decades ago. "Sex" is the correct term. There is the male sex and the female sex.

Where things get tricky is that biology is not 100% perfect. Gravity works the same, every time. An apple falls off the tree to the ground. Every time. It never falls off a tree and floats in the air, or rockets into outer space. It simply falls to the ground.

Physics is like that. Biology is not.

In biology, there are mutations and unusual occurrences. Sometimes, a two-headed snake comes along. 99.999% of the time, things are more or less "normal." But not always.

After conception, the default sex for humans is female. If the Y chromosome is present, the ovaries turn into testes, and the clit turns into a penis. There are a few (very few) humans where that transformation does not occur normally, and some sort of mutation happens instead. That is very unfortunate for those people, but that does not change the fact that there are two (and only two) sexes, with a very few people having an incomplete transition.

This is a totally different phenomenon than a person who is fully formed but grows up and has a psychological malfunction, either believing or pretending to believe that they are the "wrong" sex. Not possible to be the "wrong" sex, but entirely possible to be delusional or deceitful.

The word "gender" was introduced into the vocabulary and intended to be a fuzzy term to confuse matters. That word should not be used anymore. The correct term is "sex" (the noun, not the verb).