haploid

Haploid describes cells or organisms that contain one complete set of chromosomes, denoted as n. In eukaryotes, a haploid cell carries one copy of each chromosome; in species with more than one chromosome set, this single set is still referred to as a haploid or monoploid state. The haploid chromosome number, n, varies among species (for example, humans have n = 23; chickens have n = 39).

In sexual reproduction, haploid cells are typically produced by meiosis from diploid germ cells. These haploid gametes fuse during fertilization to form a diploid zygote, which then develops into a new organism. This cycle restores the organism’s characteristic chromosome number.

Life cycles differ across groups. Most animals have a predominantly diploid life cycle, with haploid gametes serving solely as reproductive cells. Many fungi and some algae exhibit a prominent haploid phase, where the multicellular organism is haploid and meiosis occurs after a brief diploid stage. Plants show an alternation of generations, with both multicellular haploid (gametophyte) and multicellular diploid (sporophyte) phases in some lineages.

Other notes include that bacteria and many other prokaryotes are effectively haploid, possessing a single circular chromosome, though they reproduce asexually and do not form gametes in the same sense as eukaryotes. In practical terms, haploid refers to the presence of one chromosome set, in contrast to diploid (two sets) or polyploid organisms (more than two sets).