There are several cultures that have arisen independently and have had more than two genders. Many of them are still extant today, despite the best effort of colonialists.
So that's gender down.
Intersex individuals exist. Intersex conditions range from physiological to karyotypes with variant sex chromosomes.
Transgender individuals are very varied but there are indications that it is partially heritable with genetics and the prenatal environment hypothesized to explain some of that.
And that's biology.
I could write a 100 page essay about how loaded your question was, the implicit assumptions in the way it was written, the way that "BIOLOGICAL" is implied to be a way to ignore social conditions, and the contempt for ""Gender Studies"". But I won't, it's very tiring and there will be 10 other people like you tomorrow.