The genetic handbook of the male

BBC News Online's Helen Briggs sifts through the genetic manual of the male

The world's men can utter a communal sigh of relief. According to scientists who have deciphered the Y chromosome, they have at least 50 million years left on the planet.

Men are XY: They have one Y and one X chromosome

There had been concern that the bundle of DNA that determines the male was shrinking and would be defunct within five million years.

But rumours of its demise have been dismissed. Steve Rozen, one of the leading scientists on the project, says the Y chromosome has a robust life expectancy.

He told BBC News Online: "I don't think the Y chromosome in humans is going to be lost, I would guess even in 50 or 60 million years, if we are lucky enough to survive as a species that long."

So why all the fuss about the unveiling of this chunk of genetic material that defines the male of the species? For a starter, it is the first Y chromosome of any living thing to be fully sequenced.

That means we now know every single DNA letter in the chromosome, a sort of handbook of male hardware. It is bound to fuel the thorniest of debates - how different are men and women under the skin?

Ascent of man

As it turns out, men have an extra brain gene that females do not possess. Nobody knows what it does yet - although it is bound to attract stereotypical jokes about parking and map-reading.

On the serious side, the gene must have played a key role in the evolution of mankind for it to exist in the genome today.

SEX AND OUR CHROMOSOMES

Males have one X and one Y chromosome, whereas females have two Xs

The Y chromosome is passed, almost unchanged, from father to son

It is the smallest of the 24 distinct structures that bundle up human DNA and carries only 78 genes

"It is in a part of the Y chromosome that was acquired in the last few million years since humans and chimpanzees split," says Rozen, of Massachusetts Institute of Technology in Cambridge, US.

He speculates the gene provided some sort of evolutionary advantage in the path from chimps to modern humans.

Chimps parted company from humans more than five million years ago. The Y chromosome itself has been around much longer - some 300 million years. During this time, it has fought an ancient battle of the sexes with the X chromosome.

Now and again, genes on the Y are "overwritten" by similar genes on the X. This has cost the Y hundreds of genes over the years. But the way the genes are arranged means that if a critical gene is damaged, it has a partner it can swap with.

"This study shows the Y chromosome has become very efficient at preserving its important genes," says Richard Wilson, director of the Genome Sequencing Center at the Washington University School of Medicine in St Louis.

"It's found different ways to do the things chromosomes must do to evolve, survive and thrive."

Male infertility

The science is a small blow for male pride, however, as the Y has been found to be somewhat under endowed, containing just 78 genes.

The tally is more than originally believed but rather paltry compared with the thousands of genes on other chromosomes.

THE TOUGHEST CHROMOSOME

Why the Y was so hard to decipher

Read more

So does size matter? When it comes to the Y, it seems not. It is no surprise that most of these genes are the instructions the body needs to produce sperm. The function of many of the 18 others is as yet unknown.

One is the sex determining gene, the "master switch" that makes a baby boy; three specify protein-making machinery and another is the gene that has some sort of function in the brain.

As for the others, it is currently anyone's guess. What is probable is that they hold hidden treasure for those seeking to diagnose and treat male infertility.

Genetic wasteland

The Y chromosome has been slow to give up its secrets. It wasn't until the early 20th Century that studies with the humble electron microscope first confirmed its existence.

Fifty or so years later, the advent of molecular biology brought new powers of investigation. An understanding of the chromosome's biological functions started to emerge, and with it clues to why some men have very low sperm counts.

Among the flotsam and jetsam of each chromosome lie clues to our history

Decoding the Y has proved to be an onerous task. It contains stretches of DNA that appear to be a genetic wasteland devoid of genes.

As the latest chapter in the human genome shows, the secrets of hundreds of millions of years of human evolution are buried within this seemingly meaningless "junk DNA".

"Piecing together these events remains a worthwhile challenge," says Huntington Willard of the Institute for Genome Science and Policy, at Duke University, Durham, North Carolina, "for among the flotsam and jetsam of each chromosome lie clues to our history."