Having a third copy of a tumour suppressor gene might provide extra insurance against cancer. Apart from its involvement in Down's syndrome, chromosome 21 is replete with genes of medical interest. Perhaps the most intriguing of the Consortium's findings is that chromosome 21 contains remarkably few genes. It is 33.8 'megabases', or million base pairs, long and has 225 genes in its of DNA sequence. Compared with 22, chromosome 21 looks like a genomic desert.
Large swathes appear to do nothing.
Escherichia coli , containing precisely one gene. Indeed, extrapolating from the average gene density of both sequenced chromosomes, researchers have now revised their initial estimate for the total number of human genes from the initial 70,000-140,000 down to something closer to 40,000. The paucity of genes on chromosome 21 is itself interesting. The fact is in chromosome 21, there is relatively little to go wrong. The sequencing of chromosme 21 may be the last milestone before the complete human genome sequence is published.