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Haploid and Monoploid

Haploid and Monoploid

The haploid number is the number of chromosomes in a gamete of an individual. This is distinct from the monoploid number which is the number of unique chromosomes in a single complete set.

In humans, the monoploid number (x) equals the haploid number (the number in a gamete, n), that is, x = n = 23. In some species (especially plants), these numbers differ. Commercial common wheat is an allopolyploid with six sets of chromosomes, two sets coming originally from each of three different species, with six copies of chromosomes in each cell. The gametes of common wheat are considered as haploid since they contain half the genetic information of somatic cells, but are not monoploid as they still contain three complete sets of chromosomes from the original three different species (n = 3x).

Most fungi and a few algae are monoploid organisms, and male bees, wasps, and ants are haploid because of the way they develop from unfertilized, haploid eggs. The Australian bulldog ant, Myrmecia pilosula, a haplodiploid species has n = 1, the lowest known (and lowest theoretically possible) n. A monoploid cell is likely to be identical to the cell it was copied from; however, in haploid cells one of two differing copies of the same chromosome is in the haploid set.

Plants and some algae switch between a haploid and a diploid or polyploid state, with one of the stages emphasized over the other. This is called alternation of generations. Most diploid organisms produce monoploid sex cells that can combine to form a diploid zygote, for example animals are primarily diploid but produce monoploid gametes. During meiosis, germ cell precursors have their number of chromosomes halved by randomly "choosing" one homologue, resulting in haploid germ cells (sperm and ovum).

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