copy number variation

Copy number variation describes differences between people in how many copies they carry of a particular stretch of DNA. Where one person might have the usual two copies of a segment — one from each parent — another might have one, three, or more. It is variation in dosage rather than in the spelling of individual letters.

These segments can be thousands to millions of bases long and may contain whole genes. They arise when stretches of DNA are duplicated or deleted, often through mistakes in recombination between similar repeated sequences. Copy number variants are a common and normal part of human genetic variation, with most people carrying many of them harmlessly.

Dosage can matter, though. Having too many or too few copies of certain genes is linked to specific disorders and can influence traits and disease risk. Copy number variation thus sits at the boundary between the small-scale mutations measured base by base and the large chromosomal changes studied in cytogenetics.