Chromosome 21 mapped

      Scientists have mapped chromosome 21, the smallest human chromosome and the one associated with Down syndrome, epilepsy, Lou Gehrig's disease and Alzheimer's.

It is the second human chromosome whose DNS has been mapped. Chromosome 22 was mapped laster year.

The German- and Japanese-led team that mapped both chromosomes is part of the Human Genome Project, an international effort to decipher chemically the entire human genetic blueprint. Humans are normally born with 23 different pairs of chromosomes, which are made up of genes. The researchers found that chromosome 21 contains far fewer genes ( 225 ) than in chromosome 22 which has 545 genes.

It is thought that the total number of genes in human DNA is under 40,000 and not the 100,000 or more that had previously been thought.

The mapping of the chromosome may allow researchers to home in on specific genes in the chromosome that cause mental retardation in people with Down syndrome and then perhaps develop drugs to treat such patients.

Down syndrome occurs when a person is born with an extra, third copy of chromosome 21. Down syndrome, the most common form of genetically caused mental retardation, occurs in about one in 700 live births. Down syndrome can also cause congenital heart disease and Alzheimer's by age 40.

Chromosome map could eventually shed light on why people with Down syndrome have a very low rate of breast, lung and gastrointestinal cancers. It could be that an extra copy of chromosome 21 has tumor-suppressing qualities.