diploidy

Diploidy is the condition of having two complete sets of chromosomes in the somatic cells, designated 2n. In diploid organisms, chromosomes exist as homologous pairs, one chromosome from each parent. The diploid state provides two alleles for many genes and creates a substrate for genetic variation through recombination and sexual reproduction.

In most sexually reproducing organisms, diploidy is maintained by mitotic cell divisions for growth and tissue maintenance. Gametes, such as sperm and eggs, are typically haploid (n) and produced by meiosis, a reductive division that halves the chromosome number and promotes genetic diversity through crossing over and independent assortment. Fertilization, the union of two haploid gametes, restores the diploid state in the zygote and initiates development.

Genetically, a diploid individual carries two alleles for each gene, which can be identical (homozygous) or different (heterozygous). Heterozygosity allows recessive alleles to be masked in the phenotype and enables dominance relationships and gene interactions to influence traits. Abnormalities in chromosome segregation during cell division can generate aneuploidies, such as trisomy or monosomy, with potential developmental consequences.

Diversity of life cycles accompanies diploidy. In many animals, the diploid phase is the dominant life stage. In plants and some algae, life cycles often feature alternation of generations, including both diploid sporophytes and haploid gametophytes. Some fungi are predominantly haploid or display varying degrees of diploidy during their life cycles.