Polyploidy

The presence of more than two sets of chromosomes in a cell because of a genome mutation

Organisms with three sets of chromosomes in their cells are called triploid, those with four sets are called tetraploid and those with six sets hexaploid, etc.

In certain circumstances polyploidy can occur during meiotic division (meiosis). During meiosis, gametes are produced when the cell divides and the number of chromosomes is halved. If meiosis is disrupted by external events, the cells divide but the chromosomes do not.
Polyploidy occurs in a large number of plants and often leads to an increase in output. For this reason, the phenomenon is used in plant breeding. It is possible to bring about polyploidy in plants using artificial means, e.g. cold treatment, X-radiation or by treating with colchizine (meadow saffron toxin).

Allopolyploidy is a special form of polyploidy (from the Greek allos = other). Here, replication of the chromosome set occurs when different species are combined. Oilseed rape is an allopolyploid, the result of a cross between turnip rape and cabbage (both diploid).