Duplication: Part of a chromosome in duplicate, a particular kind of mutation (change) involving the production of one or more copies of any piece of DNA, including a gene or even an entire chromosome.
Duplications typically arise from an event termed unequal crossing-over (a kind of recombination) that occurs between misaligned homologous chromosomes during meiosis (germ cell formation). The chance of this event happening is a function of the degree of sharing of repetitive elements between two chromosomes. The recombination products of such an event are a duplication at the site of the exchange and a reciprocal deletion. A remarkable class of duplications in which the duplicated region has popped up far away from home base has also been discovered.
Duplications have been important in the evolution of the human genome (and the evolution of the genomes of many other organisms).
A duplication is the opposite of a deletion.