= Copy number variation
<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 $3/2$, or $\log_2(3/2)\approx0.585$ 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>.<Large-scale copy number polymorphism in the human genome by Jonathan Sebat and others>[Abstract and genomic-hybridization study]{ref}
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