Ploidy indicates the number of copies of the basic number of chromosomes. The number of basic sets of chromosomes in an organism is called the monoploid number (x). The ploidy of cells can vary within an organism. In humans, most cells are diploid (containing one set of chromosomes from each parent), though sex cells (sperm and oocytes) are haploid. In contrast, tetraploidy (four sets of chromosomes), a type of polyploidy, is not uncommon in healthy plant species.
Euploidy, or the euploid number is the normal number of chromosomes within a cell for a species, for example the euploid number of chromosomes in a human cell is 46.
Most fungi, and a few algae exist as haploid organisms, male bees, wasps and ants are also haploid. For organisms that only ever have one set of chromosomes, the term monoploid can be used interchangably with haploid.
Plants and other 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 haploid sex cells that can combine to form a diploid zygote, for example animals are primarily diploid but produce haploid 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).
Diploid cells (meaning double in Greek) have two copies (homologs) of each chromosome (both sex- and non-sex determining chromosomes), usually one from the mother and one from the father. Most somatic cells (body cells) of higher organisms are diploid.
A haplodiploid species is one in which one of the sexes has haploid cells and the other has diploid cells. Most commonly, the male is haploid and the female is diploid. In such species, the male develops from unfertilized eggs, while the female develops from fertilized eggs: the sperm provides a second set of chromosomes when it fertilizes the egg.
Haplodiploidy is found in many species of insects from the family Hymenoptera, particularly ants, bees, and wasps. It increases the significance of kin selection, which may explain the eusociality of these sorts of insects.
Haploidisation (from the Greek απλοποίηση = simplification) is the process of creating a haploid cell from a diploid cell. This is a laboratory procedure that forces a normal cell to spit out half of its chromosome content, leaving just one set. In mammals this renders this cell equal to sperm or egg.
Aneuploidy is when a cell contains an abnormal or non-integer ploidy number. This may lead to problems in cell development. Most forms of aneuploidy in humans are lethal, but trisomy (three copies) of the sex chromosome (the cause of Klinefelter's syndrome and others) and of chromosome 21 (the cause of Down syndrome) are relatively common.
Polyploidy is the state where all cells have multiple pairs of chromosomes beyond the basic set. Polyploidy is common in plants, and rare in animals.