Gender divison in two opposites is greatest mistake of mankind

In humans, and I do believe all or most mammals, female individuals have two X chromosomes (XX), while male individuals have a Y chromosome and one X (XY).

In some species sex is not determined by the Y chromosome, but instead by the number of X chromosomes, or other factors; some fish can change sex due to environmental cues.

Some organisms, such as birds, snakes, and butterflies, have X and Y type chromosomes but the role is somewhat reversed, and instead they are called referred to as Z and W chromosomes, for X and Y respectively. In these organisms, the male has ZZ, and the female has ZW chromosomes. The "female Y" chromosome here is referred to as a "W" chromosome, I suppose since the Y chromosome is conventionally associated with male development. Of course this is somewhat arbitrary, and determining which sex to call "male" and which to call "female" in non-mammalian species may as well be arbitrary also in some cases; I can imagine that in some organisms this isn't completely clear, and which one you called male, or female, may not make too much difference except for convention.

In reptiles (and some other organisms) sex determination is through environmental factors, such as incubation temperature &c, as these organisms lack a Y (or analogous) sex chromosome.

Obviously the [human] Y chromosome has some genes which are not on the X chromosome, but otherwise contains an extreme few genes compared to other chromosomes in the human genome and very few traits are Y-linked, that is, it doesn't appear the Y chromosome itself does all that much in determining most human characteristics. The X chromosome is a lot more important and I don't think you can exist without the genes on the X chromosome; other than that, it seems much more important not to have too many of either X or Y chromosomes (or any chromosome for that matter); less than or greater than two X chromosomes (in the absence of a Y chromosome), or more than one Y chromosome will cause abnormalities, e.g. 46,X or 46,XXY, 46,XYY karyotypes will have marked abnormalities.)

However XX [human] males do exist, in certain cases the SRY (sex determining region Y) of the Y chromosome can cross over to an X chromosome leading to an XX individual that develops as a male due to those genes in the SRY portion that have crossed over.

Wow, science this early in the morning; I'll link the wikipedia page on the Y chromosome in case anyone cares to look further and maybe go to the primary sources.

https://en.wikipedia.org/wiki/Y_chromosome

 

How exactly do "women become men" with age?

 

I disagree that male and female are "opposites", they are largely similar in most respects to one another and are just slightly varied versions of the same thing.