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X-inactivation

August 11, 2009

Continuation of the unfinished post Women are Winning
The other advantage the girls have over the boys is the fact that they have two X chromosomes, whereas the boys only have one.

This is a pretty basic principle: if you have two of something, chances are it is better than one. For example, diseases and conditions that lie on the X chromosome have no choice but to create an effect in men (i.e. male pattern baldness) while women require the two copies on both chromosomes. So to break that down again, every gene in our body has two copies, one on each homologous chromosome. Homologous chromosomes are pairs of chromosomes with similar genes and are what we inherit, one from mommy and one from daddy. Except boys get one X chromosome from mommy and one Y chromosome from daddy, thus giving you a pee-pee. Girls on the other hand get an X from mommy and an X from daddy, his only one.

So what that means is, if daddy or mommy have something horrible, like baldness, being passed down in their X chromosomes, girls have a better chance of fighting it, whle boys can only hope to dodge it. If daddy is bald, he has no choice but to pass his baldness gene on to his daughter (recall he only has one X to pass on), but as long as mommy’s head is full of hair and passes her beautiful hair gene on, the one copy of beautiful hair can compensate for the baldness gene (more on this later). But sonny boy may not be so lucky. If daddy has beautiful thick hair, well sucks for him it doesn’t matter because daddy is giving him a Y chromosome instead (but if daddy has beautiful hair, then that means his daughter will also have beautiful thick hair because he must giver her that wonderful X). But if mommy has that baldness gene in one of her X’s, he has a 50% chance of being bald, all dependent on which X he gets from mommy.

So to dispel that rumor for boys: baldness is inherited from the mother, but just because granddad is bald, does not mean you will too, and just because he has gorgeous hair does not mean you may not get it either. But ultimately, the advantage is with the girls: by having two X chromosomes, they must inherit the baldness gene from both dad and mom, making the likelihood a bit lower.

But the story doesn’t just end here, and girls have even more to look forward to. Turns out, they only actually need one X chromosome! In a crazy turn of events, female cells actually turn off one X chromosome by turning it into heterochromatin, in other words, packaging the DNA really really tight so it can’t be read, like stuffing books into a closed box and instead of leaving it on the shelf. This is called X inactivation. Early on in development, each cell turns off one X chromosome, making it pretty much useless while the other X chromosome carries out all other duties, and thus making those genes the ones that are used. And which X chromosome turns off? Well it’s pretty much random! This leads to a mosaic in the female body, some cells have one X in use while a cell right next to it may have the other X. So guess what, because this is random, it would actually be impossible to completely clone a female. While creating an exact clone of anyone is pretty much impossible, what with environmental stimuli playing such an important role in development, but from a genetic profile, cloning women are even more difficult because there’s no way to get the exact X inactivation to be the same.

Where this truly plays a crucial role is in the X inherited disease, fragile X. This disease is the result of a neural gene becoming nonfunctional because of a three base pair sequence in the DNA repeating way to much, like 50+ copies instead of 1. This occurs when the DNA is being copied and the machinery literally slips and copies and replicates the same thing over again. For men, if this is passed down to them from one of their mom’s X chromosomes, they will certainly end up with mental retardation. But if a girl receives one bad copy, there is a 50% chance of mental retardation. This is because if certain important cells get the good gene and good chromosome inactivated, then they might end up with problems, but if the right cells get the bad one to inactivate, no phenotype, in this case mental retardation, can be seen. So because the female brain is a mosaic of inactivated X chromosomes, the chances to fight this disease are increased.

Coming back to this baldness gene, the female body is a mosaic of certain X chromosomes inactivated and certain ones activated. That is why girls that are heterozygous for baldness, or have one good gene and one bald gene, end up having hair. Half the cells on the scalp have the good gene while the other half have the bad. So this is enough functional hair genes to prevent baldness.

(Baldness was just used as an example, but I think female baldness is quite a bit more complicated than this. I don’t really know, and once again, I’m not a doctor…)

But does that mean women are better than men? Genetically they may be slightly more advantageous. But that kind of makes sense though, doesn’t it? Women birth our children. They are the crucial people that ensure we have another generation. Men are important too because they create the other half of the babies genes. But while women may only be able to conceive a limited number of babies, men can have as many as they want, assuming they’re players. So maybe ancient Amazonia of lore had it all right, we only need a society of women to get things done, and then the occasional man for the sake of creating a new generation. (And that guy would be really really freaking lucky).

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