ploidies

Ploidy is the number of complete chromosome sets in a cell or organism. In eukaryotes, the basic set is denoted as n, the haploid number. Somatic cells of humans are typically diploid (2n), containing two copies of each chromosome, whereas gametes are haploid (n). Polyploid refers to cells or organisms with more than two complete chromosome sets; common forms include triploidy (3n), tetraploidy (4n), and higher levels such as hexaploidy (6n). Monoploidy is used in some contexts to describe a genome with a single chromosome set, though it is rare in many lineages.

Endopolyploidy occurs when certain tissues contain polyploid cells arising from endoreduplication without cell division. Autopolyploids arise from genome duplication within a single species, while allopolyploids result from hybridization between species followed by chromosome doubling.

Aneuploidy refers to chromosome numbers that are not an exact multiple of the haploid set, such as trisomy 21. Aneuploidy typically arises from nondisjunction during meiosis or mitosis and can cause developmental disorders or contribute to cancer. Ploidy states can be dynamic in some organisms or tissues, and shifts in ploidy can influence cell size, development, and adaptation.

Detection and study of ploidy use methods such as karyotyping to count chromosomes, flow cytometry to measure DNA content, and sequencing-based approaches to infer copy number and chromosome structure. Ploidy is a fundamental concept in genetics, evolution, and plant breeding, where polyploidy frequently plays a major role, and in medicine, where aneuploidy is associated with diseases and developmental outcomes.