Just filtering out this snippet for the sake of scientific accuracy. :p There are actually
three sexes among humans (and other mammals), albeit that one of those three is relatively far less common and is considered a genetic anomaly, given that the members of that group are generally both infertile and sexually dysfunctional (in the biological sense).
- XX chromosome pairs → female
- XY chromosome pairs → male
- XO chromosome pairs → androgynous (hermaphrodite, although for humans the term "intersexual" is used)
There also exist other chromosome pairs that equally lead to androgyny, but those truly
are genetic defects, whereas in the case of specimens with
XO chromosome pairs, they are considered genetically anomalous based upon the fact that they are statistically far less prevalent than
XX and
XY chromosome pairs. And this statistical exception is of course the result of the fact that
XOs cannot naturally reproduce, which causes the genes responsible for this particular sex to vanish from the gene pool and to only reappear as a side-effect of yet
another, undefined genetic anomaly.
One particular mechanism that can (but doesn't always) lead to the birth of an androgynous mammal is the merger in the womb of the fertilized eggs of two non-identical twins before cell reproduction and differentiation begins, whereby one of the twins was male (
XY) and the other one was female (
XX).
However, as I said, this doesn't always lead to androgyny. David Bowie was not biologically androgynous ─ even though he was bisexual ─ but he had one blue eye and one brown eye, which was most likely the result of him having developed in the womb as the merger of two non-identical twins. I don't know whether Bowie ─ his real name was actually David Jones ─ effectively had his genetic makeup examined, but the difference in color between both eyes is one of the most common consequences of such a merger of non-identical twins in the womb.
Furthermore,
identical twins are not always truly identical. I grew up with a pair of supposedly identical twin girls ─ they are one year older than I am and they are the sisters of a guy who was my age and whom I went to school with. They lived only 50 meters from our door. Apart from a small mole on the forehead of one of the girls, nobody could tell them apart. In fact, I ran into one of them a short time ago and even I still mistook her for her sister, because the mole on her forehead had vanished. But one of them ─ the one with the (now vanished) mole ─ is straight and the other one is a lesbian, and sexual orientation is genetically determined. So even though they were supposedly identical twins, they do still appear to have different genes after all.
:back to topic: