The Biologian: Why Women are Chimeras and Sex Determination

Genetically speaking, what makes a male a male and a female a female? There is no uniform answer in nature:

Mammals (and some insects), as you well know, use a XX and XY system. Essentially, if you have a Y chromosome you become a male...right?...

Reptiles/amphibians/some insects, on the other hand, use a system where if you have a different chromosome than the 'normal' you become a female. In the lingo, you're male if you have WW chromosome and you're female if you have ZW. *Note, the letter has zip to do with the shape. I was told growing up that the mammal Y chromosome gets its name from the fact that it was shaped like a Y. Not true. It's a tiny 'x' and behaves the same as every other chromsome does, by crossing over during meiosis. The 'x' terminology began because in early genetics they needed to give it a name and 'x' sounds cool. ha

Some other insects have no sex chromosome, but instead the lack of an 'X' determines the sex. They're either X or XX.

Plants, fungi, protozoans and inverts don't even use sex chromosomes altogether.

Exceptions within humans:

47 XXX - female

48 XXXX - female
47 XYY -- so called supermales
48 XXXY - Extreme Klinefelters males (a male with partial female development, like breasts, etc.)
48 XXYY - Extreme Klinefelters males

Then there's even more exceptions! You can have XX males and XY females! How's that? Well, I kind of lied earlier when I said that it was the Y chromosome that make a male. It's actually a few genes on the Y and those genes can get mutated or moved over to the X chromosome. The most important controls testes development. As embryos we're all females. Then, if you have the right genes, the sex gonads get told to turn into testes. If you don't have that gene, then you develop as a female.

It gets even more awesome!

So, how do mammals deal with the problem of females having too many X chromosomes? If they displayed both of the chromosomes there could be some pretty dangerous developmental differences between males and females. Well, they turn one off! In fact, they don't just turn it off, they glom it into a blob on the side of the nucleus wrapped up in RNA and proteins in tombed so that it doesn't express. What makes this awesome is that which X is randomly chosen. It could be the one from the father or it could be the one from the mother.

Translation: female mammals are mosaics! Some of their cells use one X chromosome and some cells use the other! Mammal males are roughly all the same genetically across their cells, but females aren't! Their cells use different DNA! In fact, you can have whole patches that use one X and other patches using the other. This is called mosaicism, for obvious reasons. In kitty cats this produces some fun fur patterns that are -only- in the females. Basically, the dad had one color X and the mother another X and the baby displays both at once, but in different patches! (Why patches? Why not a blend of different cells?...Dunno.)

Barr Body in nucleus:

Mosaicism in kitties:

This one could have happened other ways, but most likely was due to mosaicism. One hint could come from if this cat is female.

Humans still have this. For us the patches don't usually manifest as a calico/tortoiseshell appearance, but instead can have effects like ectodermal displaysia, which is the patching of working and nonfunctional sweat glands:

There are other kinds of mosaicism, like that of so called chimeras, which are the result of fraternal embryos fusing together. What makes this particularly wild is that this has been an issue with welfare genetic testing and for organ transplants. One mother was accused of being a surrogate mother for someone and committing welfare fraud when a DNA test showed her as not being the mother of her kids (another article). Another instance showed that a kidney transplant couldn't go through since the relatives weren't close enough genetically.