As discussed in the introduction to this chapter, some organisms (such as bananas) possess more than two sets of chromosomes—they are polyploid. Polyploids include triploids (3n), tetraploids (4n), pentaploids (5n), and even higher numbers of chromosome sets.

Polyploidy is common in plants and is a major mechanism by which new plant species have evolved. Approximately 40% of all flowering-plant species and 70% to 80% of grasses are polyploids. Polyploid plants include a number of agriculturally important plants such as wheat, oats, cotton, potatoes, and sugarcane. Polyploidy is less common in animals but is found in some invertebrates, fishes, salamanders, frogs, and lizards. No naturally occurring, viable polyploids are known in birds, but at least one polyploid mammal—a rat in Argentina—has been reported.

We will consider two major types of polyploidy: autopolyploidy, in which all chromosome sets are from a single species, and allopolyploidy, in which chromosome sets are from two or more species.