Since the dawn of humanity, men have played a vital role in determining the sex of their offspring.
The Y chromosome, carried by roughly half of a man’s sperm, dictates whether a child will be male or female.
If the Y chromosome is present, a child will develop into a boy, whereas a lack of this strand of DNA will result in a girl.
Now scientists think the Y chromosome may disappear in less than five million years, leaving the future of men on planet Earth uncertain.
In an article for The Conversation, Dr Peter Ellis and professor, Darren Griffin from the University of Kent discuss the implications of this genetic shift.
The Y chromosome may be a symbol of masculinity, but it is becoming increasingly clear that it is anything but strong and enduring.
Although it carries the ‘master switch’ gene, SRY, that determines whether an embryo will develop as male (XY) or female (XX), it contains very few other genes and is the only chromosome not necessary for life.
Women, after all, manage just fine without one.
What’s more, the Y chromosome has degenerated rapidly, leaving females with two perfectly normal X chromosomes, but males with an X and a shrivelled Y.
If the same rate of degeneration continues, the Y chromosome has just 4.6 million years left before it disappears completely.
This may sound like a long time, but it isn’t when you consider that life has existed on Earth for 3.5 billion years.
The Y chromosome hasn’t always been like this.
If we rewind the clock to 166 million years ago, to the very first mammals, the story was completely different.
The early ‘proto-Y’ chromosome was originally the same size as the X chromosome and contained all the same genes.
However, Y chromosomes have a fundamental flaw.
Unlike all other chromosomes, which we have two copies of in each of our cells, Y chromosomes are only ever present as a single copy, passed from fathers to their sons.
This means that genes on the Y chromosome cannot undergo genetic recombination, the ‘shuffling’ of genes that occurs in each generation which helps to eliminate damaging gene mutations.
Deprived of the benefits of recombination, Y chromosomal genes degenerate over time and are eventually lost from the genome.
Despite this, recent research has shown that the Y chromosome has developed some pretty convincing mechanisms to ‘put the brakes on’, slowing the rate of gene loss to a possible standstill.
For example, a recent Danish study, published in PLoS Genetics, sequenced portions of the Y chromosome from 62 different men and found that it is prone to large-scale structural rearrangements allowing ‘gene amplification’ – the acquisition of multiple copies of genes that promote healthy sperm function and mitigate gene loss.
The study also showed that the Y chromosome has developed unusual structures called ‘palindromes’ (DNA sequences that read the same forwards as backwards – like the word ‘kayak’), which protect it from further degradation.
They recorded a high rate of ‘gene conversion events’ within the palindromic sequences on the Y chromosome – this is basically a ‘copy and paste’ process that allows damaged genes to be repaired using an undamaged backup copy as a template.
Looking to other species (Y chromosomes exist in mammals and some other species), a growing body of evidence indicates that Y-chromosome gene amplification is a general principle across the board.
These amplified genes play critical roles in sperm production and (at least in rodents) in regulating offspring sex ratio.
Writing in Molecular Biology and Evolution recently, researchers give evidence that this increase in gene copy number in mice is a result of natural selection.
On the question of whether the Y chromosome will actually disappear, the scientific community, like the UK at the moment, is currently divided into the ‘leavers’ and the ‘remainers’.
The latter group argues that its defence mechanisms do a great job and have rescued the Y chromosome.
But the leavers say that all they are doing is allowing the Y chromosome to cling on by its fingernails, before eventually dropping off the cliff.
The debate therefore continues.