Women Are Mosaics

, there is usually a good copy on the other X. So half of the cells responsible are expressing the appropriate gene, which is enough for the clotting system to at least function. Guys aren't so lucky - if your mother gave you a 'bad' X chromome, you've had it.  And it's not just heamophillia - colour blindness, some types of muscular dystrophy and a syndrome that means that you can't eat fava beans (Hannibal Lecter was clearly not a sufferer).  So are there any genes that exist on the Y chromosome but not on the X (other than the obvious!)? Well, we guys are occassionally blessed with the 'hairy ears' gene.  All of this can, of course, become much more complciated. Sufferers of the somewhat seedy sounding 'XXX Syndrome' have, you guessed it, three X chromosomes. Which means that two inactivated X chromosomes exist in every nucleic cell!

So I came across this the other day when I was flicking through ‘Molecular Biology of the Cell’ (a fantastic textbook, with the one critisism that I ALWAYS get distracted by something else within its pages ). Anyway, apparently the cells of all female mammals have a bit of an identity crisis going on – half with one way of going about things, half with another. Lets take humans as an example. We each of us have 23 pairs of chromosomes making up our genetic code in DNA (one from mum and one from dad). Now, most of these are taken up encoding some pretty important stuff, like how to run our immunesystems or blueprints for neurotansmitters in the brain. But one of these pairs is responsible for all of the unrelenting horror of Friday nights at your local Llyods No. 1 Bar – the sex chromomes. Girls get two X chromosomes while boys get just one X and a tiny little scrap of genetic information that we call a Y. And this is the crucial part: in the mammalian version of the XY sex-determination schema, females get twice as much X chromasome material as males do. But while the Y chromosome almost exclusively deals with turning an embryo into a male, the X chromosome is host to a whole load of extra genes (regions of DNA that code for particular functions) – hence their diffences in size.

Humam female cell showing the Barr Body

Humam female cell showing the Barr Body

Double the X in females than in males. What’s a girl to do? The body can’t have twice as many of these gene products floating about in one half of the species than the other, but with the same desired overall effect. So in the early stages of embryo development, the cells decide at random to swith one copy off. This leads to patches of cells running on dad’s copy of X and patches running on mums. If you take a look any female mammal cell with a nucleus (where alll of the genetic material is housed), you can see the deactivated chromosome as a black dot upon appropriate staining, called the Barr Body. But boys all run off the same copy of the X chromosome inherited from their mother – with some pretty big consequences …Heamophillia is a blood clotting disorder that affects around 1in every 5,000 – 10,000 men – essentially the body’s normal mechanism for stopping a bleed by coagulation is impaired and scabs don’t form. As you can imagine this is pretty serious! Now, the reason that it almost exclusively affects men is because it’s one of these genes coded for by the X chromosome, but is not on the Y. In female mammals, even if one of their X chromosomes has a duff gene coding for these clotting agents , there is usually a good copy on the other X. So half of the cells responsible are expressing the appropriate gene, which is enough for the clotting system to at least function. Guys aren’t so lucky – if your mother gave you a ‘bad’ X chromosome, you’ve had it.

Heterochromia is a condition whereby an individual has differing iris colours, which can be caused by X-chromasome inactivation.

And it’s not just heamophillia – colour blindness, some types of muscular dystrophy and a syndrome that means that you can’t eat fava beans (Hannibal Lecter was clearly not a sufferer) are all carried on the X-chromasome and are hence sex-linked diseases (not to be confused with a sex-transmitted disease).So are there any genes that exist on the Y chromosome but not on the X (other than the obvious!)? Well, we guys are occassionally blessed with the ‘hairy ears’ gene. All of this can, of course, become much more complciated. Sufferers of the somewhat seedy sounding ‘XXX Syndrome’ have, you guessed it, three X chromosomes. Which means that two inactivated X chromosomes exist in every nucleic cell!
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