Hi Gerri,

I’m afraid most of my info is from reading a few of my wife’s textbooks, and lots of nerding out with doctors, but Wikipedia is a pretty good place to start. The six chromosomal genders are thus:

XX: standard female
XXX: Tends to be slightly taller, only has girls. You can’t tell with the naked eye if you have it, is likely to be VASTLY underdiagnosed as we don’t usually chromosomally type people. Estimated to be about 1 in 1,000. Considered by some to be a normal phenotype for human genders.
XO: Turner Syndrome. Second X chromosome is damaged. Usually short, and sterile, but present as women. Prone to heart problems. Not usually considered a chromosomal gender but rather a problem, since it happens when the second X chromosome is damaged somehow.
XY: Standard male
XYY: Only have sons. Also about 1 in 1,000 people. Some indication they are slightly more aggressive, and there’s a SLIGHT increase of XYY in prison populations, but considered by some to be a normal phenotype for human genders. They tend to be slightly taller and grow a little faster than their XY counterparts.
XXY: Also called Klinefelter Syndrome. Most people who have it don’t know- the only real issue is sterility, but fertility treatments work. Present as male. About 1 in 500 births.

There’s some other chromosomal odd matches out there, but they tend to 1. have some serious problems associated with them (the sort of thing that my doctor friends say fall outside the normal range of human chromosomal expression and into disorder territory- think of it as the difference between XYY, where most people who have it have zero idea they are XYY, to down syndrome, where there’s a host of problems involved), and 2 be a lot more rare.

We don’t have a solid read on the numbers of people who are not XX or XY because many of them may be completely asymptomatic, ie normal, and we don’t really screen for it. Of the people who do have issues, a lot of it is fertility issues, and even THEN they don’t check until they’ve run through a bunch of other tests first.

When you get to how the endocrine system handles gender, and things like androgen insensitivity or hypersensitivity disorders, you get a much, much, much more complicated picture of the biology of gender- instead of it being a yes/no, it looks more like a double bell curve, with people all the way on one end (I’d say for the male side it’d be someone XYYY with a sensitivity to androgens), small number but there, then gradually to more and more typical biologically male presentation, a dip in the curve towards more androgynous biology/intersexed, then a rise to more standard female presentation and trailing off as you hit XXX or XY with severe androgen insensitivity.

That’s before you even get to things like intersexed people, which is a normal if somewhat rare form of biology. It also looks like in the victorian era, there were a lot more intersexed people who ended up married and with kids- the early surgeries that are now VERY controversial to try to ‘normalize’ the genitals of intersexed children are actually likely to sterilize them.

Then you have a population in the Dominican Republic that presents as girls until puberty, then they change into males- They’re called Guevedoces and it turns out plenty of them go on to have kids of their own later. Most of them identify as boys with girl bodies until puberty hits and they grow a penis. This seems to be because of an enzyme issue, which isn’t even endocrine in nature.

Each of these issues is fairly rare taken individually, but when you start adding them up, it turns into a decent slice of humanity.

So, gender as Male/Female is a gross oversimplification of the biology. There’s a LOT of middle ground and grey area.