Down's syndrome genetic blueprint found

Scientists have unravelled the part of the human genetic blueprint that causes Down's syndrome, one of the most common forms of mental handicap. This should boost research into a disorder which affects one in 700 people.

The research involves the identification of each of the 33.5 million DNA steps that go to make up chromosome 21. Down's syndrome occurs when a child is born carrying an extra copy of chromosome 21, but it is not known why having the unnecessary copy leads to the handicap.

This is the second chromosome to be mapped by scientists. The sequence of number 22 was released last year.

The goal being pursued by scientists around the world is to map the entire human genome of 23 chromosomes, but a mind-boggling distance remains to be travelled before the entire genetic blueprint is laid bare to scientific study. Together, numbers 21 and 22 make up only about 2 to 3 per cent of the total human genome, which is estimated to be up to three billion steps long.

Scientists believe that once all human DNA is sequenced it will open the way to new treatments for disease, better diagnostic tests and more effective drugs. Chromosome 21 is associated with Down's syndrome and also a type of epilepsy and susceptibility to leukaemia. It may also have links to Alzheimer's disease.

The genetic sequence for chromosome 21 was published electronically yesterday by the journal Nature. The work was done by the Chromosome 21 Mapping and Sequencing Consortium, a group of 62 scientists working in Japan, Germany, France, Switzerland, the US and Britain.

"The complete sequence analysis of human chromosome 21 will have profound implications for understanding the pathogenesis of diseases and the development of new therapeutic approaches," the Nature report said.

One of the researchers' first surprises was that number 21 may have half the number of active genes compared to number 22, and they have absolutely no idea why. Number 22 has about 545 working genes, strung out along a sequence that includes long pieces of connecting "junk" DNA.

Number 21 may have as few as 225 active genes, however, and the amount of non-functional junk DNA is therefore much higher. The researchers found one very long junk DNA stretch of seven million steps. The relative scarcity of genes on chromosome 21 means that humans may have far fewer genes than expected, perhaps no more than 40,000, compared to previous estimates of between 70,000 and 140,000.

Scientists are anxious to catalogue each of the hundreds of million steps in human DNA, not for the sake of knowing them but for the hidden information they contain.

The active parts of DNA are known as genes, and genes are the factories that drive life. If you find a gene it is usually possible to discover what that gene does and what substances it produces.

Much research is now devoted to understanding the part played by genes in disease. Some disorders are caused by mutated genes. Others are caused when a gene goes out of control and produces too much of a substance or too little.