Chromosome mutations can be grouped into three basic categories: chromosome rearrangements, aneuploidy, and polyploidy (Figure 6.3). Chromosome rearrangements alter the structure of chromosomes; for example, a piece of a chromosome may be duplicated, deleted, or inverted. In aneuploidy, the number of chromosomes is altered: one or more individual chromosomes are added or deleted. In polyploidy, one or more complete sets of chromosomes are added. A polyploid is any organism that has more than two sets of chromosomes (3n, 4n, 5n, or more).