ploidy

Ploidy refers to the number of complete sets of chromosomes in a cell or organism. In most eukaryotes, a complete set is denoted n for a single set, 2n for two sets, and so on. Somatic (body) cells of many animals and plants are diploid (2n), containing two homologous chromosome sets, while gametes (eggs and sperm) are haploid (n), carrying a single set. Fertilization restores the diploid state in the zygote.

Polyploidy describes cells or organisms with more than two complete chromosome sets. Triploid (3n), tetraploid (4n), hexaploid (6n), etc. Polyploidy is common in plants and has played a major role in their evolution; it can arise by genome duplication within a species (autopolyploidy) or by hybridization between species followed by chromosome doubling (allopolyploidy). Polyploid plants often have larger cells and sometimes altered morphology and fertility. Examples include bread wheat (hexaploid), many poppies and potatoes (tetraploid), and seedless watermelons (usually triploid).

Aneuploidy is an abnormal chromosome number that is not an exact multiple of the haploid set, typically due to nondisjunction during cell division. Aneuploid conditions include monosomy (one copy of a chromosome) and trisomy (three copies). In humans, trisomy 21 causes Down syndrome, while monosomy X (Turner syndrome) and other aneuploidies have varying effects on development and viability.

Ploidy is distinct from genome size, or C-value, which measures total DNA content and can vary independently of chromosome set number due to repetitive sequences and different chromosome sizes. Ploidy affects gene dosage, development, and fertility and has important implications in agriculture, ecology, and evolution.