The draft human genome: At the double 28/2/01. By Richard Gallagher and Carina Dennis Duplications appear to have had a significant role in the evolution of the human genome.

About 5 per cent of the human genome sequence appears to have arisen from duplication of large blocks (of more than 10 Kb) within and between chromosomes.

This is a much more prevalent feature in human than in fly, worm or yeast. Duplications enable one copy of a gene to relocate to a new site where it may take on a distinct physiological function.

Duplicated regions are likely to have contributed much to the expansion of gene families in humans.

The family of genes that encode the olfactory receptors responsible for detecting smell provide an extreme example. In all there are 1000 olfactory genes scattered throughout the genome, evidence of the importance of smell to most mammals. Yet in humans, about 60 per cent are non-functional pseudogenes, illustrating our reduced dependency on smell compared with other mammals.

While duplications offer a means of enriching gene number, they can also cause problems. During the production of sperm or eggs, the enzymes that shuffle our DNA can be confused by chunks of almost identical sequence - causing deletions of large pieces of genome, which results in human disease. Examples include DiGeorge and velocardiofacial syndromes, where most patients harbour a deletion of the long arm of chromosome 22 that is thought to have been mediated by duplicated sequences flanking the deleted region.

Similarly, duplications are thought to predispose to the rearrangement and deletion in the region of chromosome 7 that causes Williams-Beuren syndrome.