The enduring mystery of Down syndrome is not the tragic symptoms suffered by the children born with it--mental retardation, heart disease, early-onset Alzheimer's--but the fact that the kids are alive at all. Down syndrome is caused by the presence of three copies of chromosome 21 rather than the standard two. Last week a team of researchers led by labs in Japan and Germany offered the first explanation of why inheriting three copies of any other chromosome causes death before or soon after birth, but three copies of 21 does not. They sequenced 21--that is, they determined the exact order of what turned out to be the 33,546,361 chemical "letters" that constitute it--and figured out how many genes those A's, C's, G's and T's spell. Number 21, they report in the journal Nature, contains a paltry 225 genes.
Chromosome 21 is only the second to be completely sequenced (No. 22 was finished last year), and geneticists expected it to have maybe twice that number of genes. It seems that children with three copies of 21 survive because they are not burdened by extra copies of very many genes. Knowing the chemical makeup of the genes that 21 does carry now gives scientists a shot at determining precisely how an extra copy causes Down.