ploidy

Ploidy is simply the count of how many complete sets of chromosomes a cell or organism carries. If a single set is one full deck of cards, ploidy tells you whether you are holding one deck, two decks, three, or more.

The common cases have names: haploid (one set, written n) and diploid (two sets, 2n). Cells with three or more whole sets are called polyploid — triploid (3n), tetraploid (4n) and so on. Humans are normally diploid, but ploidy varies widely across life, and many crop plants, such as wheat and strawberries, are naturally polyploid.

It is important to distinguish a change in ploidy, where entire sets are added or lost, from aneuploidy, where the count of just one or a few individual chromosomes is off. Whole extra sets are sometimes tolerated, especially in plants where polyploidy has driven the evolution of new species, whereas having a single chromosome too many or too few often has serious consequences. Ploidy also changes naturally during the life cycle as cells pass between haploid and diploid states.