Rare disease genes found

Four chromosomes out of 24 have now been decoded

Another human chromosome has been decoded completely by an international team of scientists.

The genetic instructions held on chromosome 14 have been published online.

It is the fourth chromosome to be fully read and is the location for more than 60 disease-causing genes, as well as two crucial immune system codes.

The research could help scientists understand why some people are more likely to develop certain rare conditions including early-onset Alzheimer's.

Instruction manual

The human genetic code, or genome, consists of about three billion pairs of chemicals known as bases.

These are arranged in 24 distinct bundles called chromosomes and are found in the nucleus of every cell in our bodies.

The Human Genome

The instructions (genes) the human body needs to function are packaged into chromosomes

This complete set of instructions is called the human genome

There are 22 numbered chromosomes plus two sex chromosomes, the X and the Y

Scientists already have a rough draft of the entire human genome.

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

In the last three years, chromosomes 22, 21 and 20 have been decoded to this level.

Mice and fish

In order to fully read chromosome 14, scientists compared its DNA with the mouse and zebrafish genomes.

The researchers found 1,050 genes on the chromosome. The chromosome itself contains 87.4 million base pairs or DNA letters.

But this is the first time that scientists believe they have left no gaps in the genetic sequence.

They have managed to find the location of not only specific genes that code for proteins, but also the start and stop signals for these genes.

Roland Heilig, from the Genoscope-Centre National de Sequencage, France, led one of the teams behind the work.

He told BBC News Online that researchers were delighted to have reached the gold standard on chromosome 14.

He said: "The identification of disease genes will help us find out how diseases develop at a molecular level."

Chromosome 14 contains two genes that are crucial in making our immune system work properly.

Genetic screening

Another 60 genes have been found that can cause disease when they go wrong.

They include genes involved in early-onset Alzheimer's and an extremely severe form of Usher syndrome, which affects hearing and vision.

Chromosome 14 is also the location for the faulty genes responsible for Niemann-Pick disease, a very rare but deadly condition.

Developing treatments, like chromosome-specific drugs that could repair the damage in the genetic code, are a long way off but this work could help doctors screen for these genetic conditions.

The research is published online on the Nature website.