Polyploidy

Polyploidy is the condition of having more than two complete sets of chromosomes in a cell. Humans and most animals are diploid, with two sets, but polyploidy is common in plants and has been an important force in the evolution of crops and wild species.

It usually follows an error in meiosis where a gamete receives a full set of chromosomes instead of half, or a failure of cell division. In autopolyploidy, the extra sets come from the same species. In allopolyploidy they come from two different species that hybridize and then double their chromosomes, which can restore fertility to a hybrid. Chemicals such as colchicine, which blocks spindle formation, are used by breeders to induce it.

Bread wheat is hexaploid, with six sets of chromosomes from three ancestral grasses. Cultivated strawberries are octoploid, potatoes are tetraploid, and the seedless banana is triploid, which makes it sterile. Polyploids are often larger or more vigorous, and many flowering plants carry traces of ancient genome duplications. Polyploidy is one form of the gene duplication that generates new genetic material.

In animals it is rare, because it disrupts sex determination, but it occurs in some fish, such as salmon and sturgeon, amphibians and insects. Because a polyploid cannot easily breed with its diploid parents, it can create a new species in one step, a mode of speciation. A different condition, aneuploidy, involves the gain or loss of single chromosomes and is usually harmful.

• Aneuploidy — abnormal numbers of single chromosomes
• Speciation — the process polyploidy can trigger
• Gene Duplication — a smaller-scale copying of genes