A cell is haploid if it contains exactly half of a species’ typical full set of genetic material. Haploid cells are often used in sexual reproduction.
In cells, ploidy defines the number of copies of a chromosome found within the cell. The number of the basic set of chromosomes is described as the monoploid number. Most cells within a human (and other animals) are diploid, which means they have two copies of each chromosome. Sex cells, however, are haploid – they have only one copy of each chromosome. This is not exactly the same as monoploidy; rather, one of two differing copies of the same chromosome is in the haploid set. A monoploid cell, however, is likely to be identical to the cell it was copied from.
In animals, haploid cells are found only in sex cells. In fungus and certain algae, however, haploid cells are the norm. Male bees, wasps, and ants are haploid because of the way they develop: from unfertilized, haploid eggs. Plants and some algae switch between stages of diploidism, haploidism, and polyploidism. Haploid cells in animals are formed through meiosis, where one chromosome is chosen at random to inhabit the haploid germ cells.
In sexual reproduction, haploid cells come together to make a diploid organism. The mixing of chromosomes provided by this encourages the evolution of species without too many wasted mutations. Without the haploid/diploid changes in cells, multicellular organisms would probably not be able to exist in the advanced forms they have today.
Web Resources On Haploid Cells
Definition of Ploidy
The Haploid Life Cycle
Book Resources On Haploid Cells
Haploids in Plant Breeding by Nitzsche & Wenzel
Androgenesis and Haploid Plants by Yves Chupeau (Editor)