Copy Number Variation
The current focus on SNPs as the new currency of genetic diversity should not divert our attention from a new source of molecular variation, the copy number variation (CNV). These have so far been studied mainly in humans and mice, but recently also in cattle and pigs. A detailed comparison of the genome of different cattle showed that hundreds of CNVs per genome involve segments of 1 kb to millions of base pairs. A sizable fraction of the CNV regions even contain functional genes, many of which are in cattle involved in environmental response. Interestingly, several reports indicate an association of human SNPs with neuropsychiatric disorders, obesity and several other pathogenic traits. So unlike most microsatellites and SNPs, CNV are not just innocent bystanders that deny causal relationship to variable traits, but are likely to be involved actively in phenotypic diversity. Other interesting observations are (1) a mutation rate of up to 10-3 per generation for a few percent of the human CNVs, (2) indications for somatic CNV variation, and (3) differences between monozygotic twins. This may suggest that CNV are a continuous source of new diversity and, challenging current concepts, that genetic variation is more than a heritage of the past. As formulated 2500 years ago by the Greek philosopher Heraclitus and his followers: Παντα ρει και ουδεν μενει [all things change and nothing remains].