Thursday, 5 June 2008

During meiosis, do the X and Y chromosomes cross over?

That's a really good question.

The principle of crossover during meiosis is that material from the same loci [the pleural of locus] on two different chromosomes can be exchanged. To borrow a metaphor from Richard Dawkins, if you consider each chromosome as a file, you will have two of each file - a set from each of your parents. Crossover entails swapping page 43, say, from your "mother's" file 2 (i.e. chromosome 2) with page 43 from your "father's" file 2. Since there are very likely small inherited differences this material - in other words, the chromosome 2 from your father is not exactly the same as the chromosome 2 from you mother - crossover generates new combinations of genes on chromosomes.

Whew! Hope that made sense. Now, the X and Y chromosomes are rather different, aren't they? Look at the picture - it's an electron microscope image of the massive X chromosome sitting next to the puny Y chromosome. (Why is the Y chromosome so small? That's a tale for another day...)

And so our initial expectation might be that they would not crossover. And we would be right. Mostly.

As it turns out though, there are actually small areas of homology between these two chromosomes. They lie at the tips of the chromosomes, and are sometimes known as pseudoautosomal regions. And, in line with our theory, since these areas are homologous, they do undergo crossover.


  1. Do you mean "the massive X chromosome sitting next.... ?

  2. Er, yes, I've changed it now!

  3. Very interesting. I used to believe they did not due to the fact that crossing over only occurs with regions that are homologus. I guess with science you can never assume.