chromosomal duplication
A chromosomal duplication is when a segment of a chromosome gets copied so the same stretch of DNA appears twice. If a deletion is a torn-out chapter, a duplication is a chapter that was photocopied and bound back in, so some genes now exist in three copies instead of the usual two. The cell then carries an extra dose of those genes.
Duplications often arise during DNA replication or recombination when repeated sequences misalign and a region is copied twice. They are a common source of copy number variation, the kind of structural change in which the number of copies of a DNA segment differs between individuals. Many such variants are harmless and part of normal human diversity.
Whether a duplication matters depends on which genes are over-dosed. Over time, duplications are also a powerful engine of evolution: a spare copy of a gene is free to mutate while the original keeps doing its job, occasionally giving rise to a new function. So the same mechanism that can cause disease also helps build new genes across generations.
Charcot-Marie-Tooth disease type 1A is most often caused by a duplication of a region on chromosome 17 containing the PMP22 gene, giving an extra gene copy.
An extra copy of a dose-sensitive gene can be just as harmful as losing one.
Duplication adds gene copies (extra dose); deletion removes them (reduced dose). Both are structural changes, and both are common forms of copy number variation.