Scientists have deciphered the make-up of the Y chromosome, the bundle of DNA that defines the male.

We can start trying to figure out how the presence of these genes controls the development to become a male

Geneticist John McPherson

The genetic data will help researchers understand the causes of male infertility, as well as the evolution and function of the Y. It turns out that large chunks of the DNA sequence are multiple repeats of seemingly meaningless stretches of genetic code.

Buried within these regions are genes that are essential for male development, including the master switch that "makes" a baby boy.

The work was carried out as part of the international human genome sequencing effort, largely by US teams.

Male 'catalogue'

John McPherson of the Genome Sequencing Centre, Washington University School of Medicine, St Louis, Missouri, US, said the genetic map of the Y would help researchers find clues to what makes a male.

The X (left) and Y chromosomes

"It gives us a complete catalogue of all the genes and we can start trying to figure out what the function is and how the presence of these genes controls the development to become a male," Dr McPherson told BBC News Online.

"We're a long way off from understanding everything but we now have the catalogue that we can look at, and these are the triggers that trigger a whole series of events in development."

With about 60 million building blocks, the Y chromosome is the smallest of the 46 chromosomes in the human body.

Window to the past

As well has helping researchers investigate the genetic basis of male infertility, the data on the Y will be useful in tracing the roots of male ancestry.

The Y chromosome is passed, almost unchanged, from father to son, but tiny variations, known as polymorphisms, occur gradually over time. This gives geneticists a tool to look back in history and trace man's earliest paternal ancestors.

The Y chromosome sequence also throws light on an ancient biological battle of the sexes between the female, X, chromosome and the male, Y, chromosome.

Males have one X and one Y chromosome, whereas females have two Xs. Occasionally, genes on the Y chromosome are "overwritten" by similar genes on the X chromosome.

The tracts of repeated sequences on the Y chromosome may be necessary to ensure that essential genes are not lost in this battle with the X.