SE - XY?

Back in primary school, everyone would tease the redhead kids because they were going to be extinct by 2030 (or some other date soonish in the future). As everyone got older we discovered that wasn’t actually going to happen (sidenote – if you weren’t aware, redheads won’t be extinct by 2030).

But recently while socialising with some of my prettier friends and discussing the shortcomings of the different genders, I heard one of them snicker to another that they’d have the last laugh as males would “soon be extinct”.

Having not pegged either of them as genocidal maniacs, I was curious to know what they were talking about. To my surprise I was told that in the next 4.5 million years the Y-chromosome would disappear and men would not exist.

Putting aside the fact that 4.5 million years was only “soon” to astronomers and geologists, I was intrigued by this. Was it true? Is the mighty Y-chromosome really going to disappear?!

(Spoilers – No, it’s not)

Some background – humans have “chromosomal sex”. Women have two medium-sized X chromosomes and men have one X and a (tiny) Y chromosome. When we do the deed and procreate mini-humans, each parent gives one chromosome to spawnling.

These chromosomes are full of different genes that perform different functions. The X has about 1,600 genes but the (not-so-mighty) Y has way less – around 50. But only 27 of these are the “you-will-have-a-penis” part of the chromosome. And many of these genes are duplicates and inactive. Honestly most of the Y-chromosome is “junk DNA” that doesn’t really do anything.

Because of this, in the early 2000s, scientists John Aitken and Jennifer Graves speculated that the Y-chromosome shows all the signs of a degraded chromosome near the end of its life.[1]

We know our sex chromosomes started as a pair of ordinary chromosomes – like in birds. Plus, sex chromosomes are ordinary chromosomes in monotreme mammals (e.g platypuses) which last shared a common ancestor with humans 166 million years ago.

This means that in the past 166 million years the Y-chromosome has lost nearly all of its 1,600 genes. That is about 10 every million years and if that continues the Y chromosome will disappear in 4.5 million years!

But this idea has two big problems. Firstly it assumes that decaying genes is bad or unusual – which it isn’t! As time passes everything breaks down, genes included. Secondly it assumes the rate of gene decay is constant.

There’s no reason to think the Y-chromosome’s decay would be constant. One could guess that the decay could get faster as the Y becomes more unstable or it might stop when the Y is stripped down to only the essential genes. The latter is exactly what scientists think has happened.

Biologist David Page’s group from the MIT Whitehead Institute are at the forefront of this research, monitoring the decay of the Y-chromosome by comparing it to other related animals. They found that since we split from monkeys 25 million years ago very few genes have been lost. And since we split from chimpanzees 5 million years ago we haven’t lost any at all (while chimpanzees have lost a few). All signs that the Y-chromosome has stopped decaying!

So my apologies ladies – seems like the Y-chromosome won’t be disappearing anytime “soon” and you’ll have to keep putting up with us men for the foreseeable future.

[1] ‘Human spermatozoa: The future of sex’, Nature 415, 963 (2002)