05 April 2011

The amazing X chromosome

I mentioned the X chromosome a while back and I just want to take a second to discuss mosaicism and how awesome it is that women are mosaic for the X chromosome.  Seriously, this is really, really cool.

As I’m sure we all remember from biology class (unless we were falling asleep like I tended to do – that was actually the only candid photo of me in my high school yearbook, me sleeping through physics)… our DNA is housed in chromosomes – 46 of them, to be exact.  We get 23 from Mom and 23 from Dad, so what we really have is two copies of each of these 23 chromosomes.  And one of them is the sex chromosome, which comes in flavors of X and Y – men get XY and women get XX.

Human chromosomes.  The sex chromosomes are outlined in red.
Note how much smaller the Y is...

Well.  Lots of species have a sex chromosome.  And there is a problem with sex chromosomes.  The X is a total gargantuan beast – it’s way, way bigger and houses a lot more genetic information than the measly Y.  It’s so much bigger, in fact, that you can essentially disregard the Y size-wise – females basically have double the dosage of sex chromosome material that men do.  We take care of this difference between sexes with dosage compensation.  How we do it, though, that’s the cool part.

So let me preface by saying not all species operate the same way.  Take insects, for example.  Male flies dosage compensate in a pretty dull way – they just transcribe twice as much RNA from the DNA on that single X chromosome, so it looks like they’ve got the same amount as females do.  Blah.  Who cares.  Boring.

Well, female mammals do a much cooler thing.  In each cell in our bodies, we condense one of our two X’s down into an impenetrable mass called a Barr body – the transcription machinery can’t access the genes on the Barr body and it’s forced to use only the remaining X.  But here’s the coolest part – beware, your mind is about to be blown! – each cell gets to choose which X to condense!  

I know, right?  This means that in a woman’s body, each of her cells is expressing either her mom’s or her dad’s X chromosome, but not both.  And once this process has occurred, it stays that way for life.

Every woman is therefore a mosaic for the X chromosome, almost like she’s two different people smashed into one – at least with regard to her X genes. 

This is totally cool.  Don’t believe me?  Check out a calico cat.  Bam. 

Calico cats are almost exclusively girls – why?  Because of our astounding Barr bodies.  The gene for coat color is on the X chromosome, and calico cats have inherited the orange version of the gene from one parent and the black version of the gene for the other.  When these lovely ladies’ cells go to work and condense one X or the other into Barr bodies, suddenly you get splotches of each color in their coats.

This image was taken directly from http://www.bio.miami.edu/dana/dox/calico.html,
which gives a great explanation of what I'm describing.

‘But I’ve seen a male calico cat, I swear,’ you tell me.  I will bet you a bazillion dollars he’s XXY. 

And this is the very best part about biology.  Nothing in biology is ever straightforward, and the only rule that doesn’t have exceptions is the one stating that every biological rule has exceptions, and that’s why I love it and think everyone in the world needs to study it.  After enough biology you give up on ever trying to defend absolutes.  Imagine how much better the world would be if we could learn from biology not to construct absolutes and defend them to the death…

Anyway, because it is biological sometimes genetics is mucky, and sometimes Mom or Dad gives you more chromosomes than you need.  And these poor calico boy kitties got two X’s and a Y.  And the presence of two X’s necessitates condensation of one or the other into a Barr body.  So these XXY boys are also calico cats (…and calico dreams…  Anyone?  Alien Ant Farm?  No?  Oh well, worth a shot).

Okay, okay, I give in, I won’t be too sexist today.  Girl mammals may be super awesome with our mosaicism and all – but boy mammals are kind of cool too, and here’s why. 

That XXY cat?  He’s a boy.  If that was an XXY fly?  She’d be a girl. 

XXY: boy kitty, girl fly. (Cat picture taken from here)

Insects determine sex by how many X’s you have.  Mammals only care about whether you’ve got a Y.  So guys, that little nothing piece of genetic dirt your daddy gave you, well, I guess it’s kind of important for some things.

P.S. – While I’m on the subject, I wrote that while-back post about red-green colorblindness.  I said the gene for colorblindness was on the X chromosome.  And I said that women, who have two X’s, are able to compensate for the loss of color discrimination in one gene by having a functional copy on their other chromosome.  But wait!

I just said women are mosaic for the X chromosome.  So, half of the color-detecting cone cells lining the back of these women’s eyeballs can detect color just fine, but half are colorblind!  Whoa, I know, I just blew your mind again.  And I didn’t give you any warning.  Sorry. 

Luckily, I’ve just kind of lied about how eyes work.  I should probably define what I mean by a ‘colorblindness’ gene. 

You have three types of cone cell – red, green, and blue – and they detect these colors using specific red, green or blue receptor proteins.  The genes required to make red and green receptors lie right next to each other on the X chromosome and they look very similar to each other in terms of sequence.  In most cases of red-green colorblindness, one of these two genes goes missing or mutates – and suddenly that receptor can’t be made, so the person has no red cones or no green cones.

In a woman who has one ‘good’ and one mutant X, half her cells trying to make green receptors are failing, but the other half are succeeding just fine.  And all the cells trying to make red or blue receptors are chugging along like normal.  So she’s got enough cells of all types to discriminate colors as usual.

The reason there are still some colorblind women is that they have inherited mutations in both of their receptor genes.  The chance of getting two mutant X genes is a lot lower than getting one, and that’s why male colorblindness is far more frequent.

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