(genetics, cell biology)
Ploidy refers to the number of sets of homologous chromosomes in the genome of a cell or an organism. Each set is designated by n. Haploidy is the state of having the half of the set of chromosomes as in a germ cell or a gamete. Its symbol is, similarly, n. The term haploid refers to a cell (or an organism) with half of the number of homologous chromosomes in somatic cells. This is used in contrast to somatic cells with diploid number of homologous chromosomes. In humans and other higher forms of living things, the diploid state of somatic cells is due to the resulting union of haploid gametes during fertilization. One of the two sets is derived from the mother's gamete and the other is from the father's gamete. In a diploid state the haploid number is doubled. Thus, haploidy occurs during gamete formation. In particular, a human germ cell (a sperm or an egg cell) is haploid, which means it contains only one of each of the 23 chromosomes of the human genome, or it only has half the diploid (2n) number of a human somatic cell (which is 46). Gametes being haploid are essential particularly during fertilization. The union of two sex cells, each with only haploid number of chromosomes, results in a diploid zygote. The integrity of chromosomal number throughout generations is eventually preserved.
Word origin: Greek haplous (single) Compare:
- haploidic (adjective)
- haploidy (noun)