Copy number variation is variation in the number of copies of a particular DNA segment among genomes. A deletion reduces the count; a duplication increases it. For an segment on a non-sex chromosome normally present once on each of two homologous chromosomes, a diploid cell usually has two copies, whereas one additional tandem copy produces a total of three.
The number must be interpreted against a specified baseline. In an ideal comparison with equal sample normalization, three copies versus two give a DNA abundance ratio of , or on a logarithmic readout. This arithmetic does not imply that gene expression must rise by the same proportion.
Genome-wide DNA hybridization experiments in 2004 found substantial large-scale copy number variation among otherwise healthy individuals. Such variation is part of ordinary human genetic diversity as well as a possible contributor to disease.[ref]
Large-scale copy number polymorphism in the human genome by Jonathan Sebat and others. Abstract and genomic-hybridization study
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Copy Number Variation (CNV) refers to a type of genetic variation where the number of copies of a particular segment of DNA varies from one individual to another. This variation can involve deletions (loss of DNA segments), duplications (extra copies of segments), or even more complex structural changes within the genome. CNVs can range in size from a few hundred base pairs to several million base pairs and can encompass one or multiple genes.