Scientists have deciphered the complete genetic instructions of a third chromosome, one of the 24 distinct bundles of DNA that carry our genetic material.

The discovery could shed light on why some people are more likely to develop common diseases such as diabetes, obesity or eczema because of their genes.

It could also give an insight into variant Creutzfeldt-Jakob disease (vCJD), the human form of mad cow disease. The gene that appears to make some people more susceptible to contracting vCJD is found on chromosome 20.

Disease 'quest'

The work is part of the ongoing Human Genome Project, an international effort to sequence (read) the genetic make-up of humans.

Dr Mike Dexter, Director of the Wellcome Trust, said: "This is one more completed chapter of our genomic anatomy textbook - medical research will be using this information for decades to come in its quest to tackle our common diseases.

"Once again, we are seeing the fruits of the two key commitments of the Human Genome Project: to make the sequence freely available and to produce a quality finished sequence."

'Gold standard'

The first draft of the entire human genome, the instructions needed to make a human being, was unveiled in the spring.

The work was carried out by two groups, the publicly funded International Human Genome Sequencing Consortium, and a private US company, Celera Genomics.

Scientists are now trying to fill in some of the gaps in the data to come up with a "gold standard".

Two human chromosomes have already been completed to this standard. The sequence of chromosome 22 was revealed two years ago. This was followed, in May 2000, by chromosome 21.

The completion of chromosome 20 is significant for a number of reasons:

• It is the largest chromosome to be finished so far. Nearly 60 million genetic "letters" have been sequenced and there are only four gaps
• It is also the first chromosome to be sequenced that has a typical structure of long and short arms
• More than 720 genes have been found. They include the genes that underpin multi-factorial diseases such as diabetes, obesity and childhood eczema
• Intriguingly, some people (37%) have an extra chunk of DNA in chromosome 20 and could have an extra copy of a specific gene of unknown function.

"The next step is to start understanding the functions of the genes," Dr Deloukas told BBC News Online.

"The sequence, the genes and the variations between people in the population is the tool to start dissecting complex common diseases like diabetes, obesity and eczema."

The chromosome 20 results are published in full in the journal Nature.