Abstract
Chromosome 21 is the smallest autosome, comprising only about 1.9% of human DNA, but represents one of the most intensively studied regions of the genome. Much of the interest in chromosome 21 can be attributed to its association with Down's syndrome, a genetic disorder that afflicts one in every 700 to 1000 newborns. Although only 17 genes have been assigned to chromosome 21, a very large number of cloned DNA segments of unknown function have been isolated and regionally mapped. The majority of these segments detect restriction fragment length polymorphisms (RFLPs) and therefore represent useful genetic markers. Continued molecular genetic investigation of chromosome 21 will be central to elucidating molecular events leading to meiotic non-disjunction and consequent trisomy, the contribution of specific genes to the pathology of Down's syndrome, and the possible role of chromosome 21 in Alzheimer's disease and other as yet unmapped genetic defects.