haploid

A haploid cell carries just one complete set of chromosomes instead of the usual pair — a single copy of the genetic library rather than a matched pair of copies. In animals, the egg and the sperm are the familiar haploid cells.

Where a diploid human cell has 46 chromosomes in 23 pairs, a haploid human gamete has 23 chromosomes, one from each pair. Geneticists write the haploid number as n; for humans n equals 23. Carrying a single set means that for each gene a gamete contributes exactly one allele to the next generation.

Haploid cells are produced by meiosis, a specialized division that halves the chromosome number so that when two gametes fuse at fertilization the diploid number is restored rather than doubled with each generation. In some organisms the haploid stage is more than a brief gamete: many fungi, algae and plants spend a substantial part of their life cycle in a haploid form.