Haploid and monoploid in predominantly haploid organisms and predominantly diploid organisms.
1) A haploid organism is on the left and a diploid organism is on the right.
2 and 3) Haploid egg and sperm carrying the dominant purple gene and the recessive blue gene, respectively. These gametes are produced by simple mitosis of cells in the germ line.
4 and 5) Haploid sperm and egg carrying the recessive blue gene and the dominant purple gene, respectively. These gametes are produced by meiosis, which halves the number of chromosomes in the diploid germ cells.
6) The short-lived diploid state of haploid organisms, a zygote generated by the union of two haploid gametes during sex.
7) The diploid zygote which has just been fertilized by the union of haploid egg and sperm during sex.
8) Cells of the diploid structure quickly undergo meiosis to produce spores containing the meiotically halved number of chromosomes, restoring haploidy. These spores express either the mother's dominant gene or the father's recessive gene and proceed by mitotic division to build a new entirely haploid organism.
9) The diploid zygote proceeds by mitotic division to build a new entirely diploid organism. These cells possess both the purple and blue genes, but only the purple gene is expressed since it is dominant over the recessive blue gene. The term
haploid is used with two distinct but related definitions. In the most generic sense, haploid refers to having the number of sets of chromosomes normally found in a
gamete. Because two gametes necessarily combine during sexual reproduction to form a single zygote from which somatic cells are generated, healthy gametes always possess exactly half the number of sets of chromosomes found in the somatic cells, and therefore "haploid" in this sense refers to having exactly half the number of sets of chromosomes found in a somatic cell. By this definition, an organism whose gametic cells contain a single copy of each chromosome (one set of chromosomes) may be considered haploid while the somatic cells, containing two copies of each chromosome (two sets of chromosomes), are diploid. This scheme of diploid somatic cells and haploid gametes is widely used in the animal kingdom and is the simplest to illustrate in diagrams of genetics concepts. But this definition also allows for haploid gametes with
more than one set of chromosomes. As given above, gametes are by definition haploid, regardless of the actual number of sets of chromosomes they contain. An organism whose somatic cells are tetraploid (four sets of chromosomes), for example, will produce gametes by meiosis that contain two sets of chromosomes. These gametes might still be called haploid even though they are numerically diploid. An alternative usage defines "haploid" as having a single copy of each chromosome – that is, one and only one set of chromosomes. In this case, the nucleus of a
eukaryotic cell is said to be haploid only if it has a single set of
chromosomes, each one not being part of a pair. By extension a cell may be called haploid if its nucleus has one set of chromosomes, and an organism may be called haploid if its body cells (somatic cells) have one set of chromosomes per cell. By this definition haploid therefore would not be used to refer to the gametes produced by the tetraploid organism in the example above, since these gametes are numerically diploid. The term
monoploid is often used as a less ambiguous way to describe a single set of chromosomes; by this second definition, haploid and monoploid are identical and can be used interchangeably.
Gametes (
sperm and
ova) are haploid cells. The haploid gametes produced by most organisms combine to form a
zygote with
n pairs of chromosomes, i.e. 2
n chromosomes in total. The chromosomes in each pair, one of which comes from the sperm and one from the egg, are said to be
homologous. Cells and organisms with pairs of homologous chromosomes are called diploid. For example, most animals are diploid and produce haploid gametes. During
meiosis, sex cell precursors have their number of chromosomes halved by randomly "choosing" one member of each pair of chromosomes, resulting in haploid gametes. Because homologous chromosomes usually differ genetically, gametes usually differ genetically from one another. All
plants and many
fungi and
algae switch between a haploid and a diploid state, with one of the stages emphasized over the other. This is called
alternation of generations. Most fungi and algae are haploid during the principal stage of their life cycle, as are some primitive plants like
mosses. More recently evolved plants, like the
gymnosperms and
angiosperms, spend the majority of their life cycle in the diploid stage. Most animals are diploid, but male
bees,
wasps, and
ants are haploid organisms because they develop from unfertilized, haploid eggs, while females (workers and queens) are diploid, making their system
haplodiploid. In some cases there is evidence that the
n chromosomes in a haploid set have resulted from duplications of an originally smaller set of chromosomes. This "base" number – the number of apparently originally unique chromosomes in a haploid set – is called the
monoploid number, also known as
basic or
cardinal number, or
fundamental number. As an example, the chromosomes of
common wheat are believed to be derived from three different ancestral species, each of which had 7 chromosomes in its haploid gametes. The monoploid number is thus 7 and the haploid number is 3 × 7 = 21. In general
n is a multiple of
x. The somatic cells in a wheat plant have six sets of 7 chromosomes: three sets from the egg and three sets from the sperm which fused to form the plant, giving a total of 42 chromosomes. As a formula, for wheat 2
n = 6
x = 42, so that the haploid number
n is 21 and the monoploid number
x is 7. The gametes of common wheat are considered to be haploid, since they contain half the genetic information of somatic cells, but they are not monoploid, as they still contain three complete sets of chromosomes (
n = 3
x). In the case of wheat, the origin of its haploid number of 21 chromosomes from three sets of 7 chromosomes can be demonstrated. In many other organisms, although the number of chromosomes may have originated in this way, this is no longer clear, and the monoploid number is regarded as the same as the haploid number. Thus in humans,
x =
n = 23.
Diploid of a typical human cell, showing a diploid set of 22
homologous autosomal chromosome pairs. It also shows both the female (XX) and male (XY) versions of the two
sex chromosomes (at bottom right), as well as the
mitochondrial genome (to scale at bottom left). Diploid describes a cell or nucleus which contains two copies of genetic material, or a complete set of chromosomes, paired with their homologs (chromosome carrying the same information from the other parent).
Diploid cells have two
homologous copies of each
chromosome, usually one from the
mother and one from the
father. All or nearly all mammals are diploid organisms. The suspected tetraploid (possessing four-chromosome sets) plains viscacha rat (
Tympanoctomys barrerae) and golden viscacha rat (
Pipanacoctomys aureus) have been regarded as the only known exceptions (as of 2004). However, some genetic studies have rejected any
polyploidism in mammals as unlikely, and suggest that amplification and dispersion of repetitive sequences best explain the large genome size of these two rodents. All normal diploid individuals have some small fraction of cells that display
polyploidy.
Human diploid cells have 46 chromosomes (the
somatic number,
2n) and human haploid
gametes (egg and sperm) have 23 chromosomes (
n).
Retroviruses that contain two copies of their RNA genome in each viral particle are also said to be diploid. Examples include
human foamy virus,
human T-lymphotropic virus, and
HIV.
Polyploidy Polyploidy is the state where all cells have multiple sets of chromosomes beyond the basic set, usually 3 or more. Specific terms are
triploid (3 sets),
tetraploid (4 sets), pentaploid (5 sets), hexaploid (6 sets), heptaploid Some higher ploidies include hexadecaploid (16 sets), dotriacontaploid (32 sets), and tetrahexacontaploid (64 sets), though Greek terminology may be set aside for readability in cases of higher ploidy (such as "16-ploid"). Ploidy of systems such as the
salivary gland,
elaiosome,
endosperm, and
trophoblast can exceed this, up to 1048576-ploid in the silk glands of the commercial silkworm
Bombyx mori. The so-called
Brassica triangle is an example of allopolyploidy, where three different parent species have hybridized in all possible pair combinations to produce three new species. Polyploidy occurs commonly in plants, but rarely in animals. Even in diploid organisms, many
somatic cells are polyploid due to a process called
endoreduplication, where duplication of the
genome occurs without
mitosis (cell division). The extreme in polyploidy occurs in the
fern genus
Ophioglossum, the adder's-tongues, in which polyploidy results in chromosome counts in the hundreds, or, in at least one case, well over one thousand. It is possible for polyploid organisms to revert to lower ploidy by
haploidisation.
In bacteria and archaea Polyploidy is a characteristic of the bacterium
Deinococcus radiodurans and of the
archaeon Halobacterium salinarum. These two species are highly resistant to
ionizing radiation and
desiccation, conditions that induce
DNA double-strand breaks. This resistance appears to be due to efficient
homologous recombinational repair.
Variable or indefinite ploidy Depending on growth conditions,
prokaryotes such as
bacteria may have a chromosome copy number of 1 to 4, and that number is commonly fractional, counting portions of the chromosome partly replicated at a given time. This is because under exponential growth conditions the cells are able to replicate their DNA faster than they can divide. In ciliates, the macronucleus is called
ampliploid, because only part of the genome is amplified.
Mixoploidy Mixoploidy is the case where two cell lines, one diploid and one polyploid,
coexist within the same organism. Though polyploidy in humans is not viable, mixoploidy has been found in live adults and children. There are two types: diploid-triploid mixoploidy, in which some cells have 46 chromosomes and some have 69, and diploid-tetraploid mixoploidy, in which some cells have 46 and some have 92 chromosomes. It is a major topic of cytology.
Dihaploidy and polyhaploidy Dihaploid and polyhaploid cells are formed by
haploidisation of polyploids, i.e., by halving the chromosome constitution. Dihaploids (which are diploid) are important for selective breeding of tetraploid crop plants (notably potatoes), because selection is faster with diploids than with tetraploids. Tetraploids can be reconstituted from the diploids, for example by somatic fusion. The term "dihaploid" was coined by Bender to combine in one word the number of genome copies (diploid) and their origin (haploid). The term is well established in this original sense, but it has also been used for doubled monoploids or
doubled haploids, which are homozygous and used for genetic research.
Euploidy and aneuploidy Euploidy (
Greek eu, "true" or "even") is the state of a cell or organism having one or more than one set of the same set of chromosomes, possibly excluding the
sex-determining chromosomes. For example, most human cells have 2 of each of the 23 homologous monoploid chromosomes, for a total of 46 chromosomes. A human cell with one extra set of the 23 normal chromosomes (functionally triploid) would be considered euploid. Euploid
karyotypes would consequentially be a multiple of the
haploid number, which in humans is 23.
Aneuploidy is the state where one or more individual chromosomes of a normal set are absent or present in more than their usual number of copies (excluding the absence or presence of complete sets, which is considered euploidy). Unlike euploidy, aneuploid karyotypes will not be a multiple of the haploid number. In humans, examples of aneuploidy include having a single extra chromosome (as in
Down syndrome, where affected individuals have three copies of chromosome 21) or missing a chromosome (as in
Turner syndrome, where affected individuals have only one sex chromosome). Aneuploid
karyotypes are given names with the suffix
-somy (rather than
-ploidy, used for euploid karyotypes), such as
trisomy and
monosomy.
Homoploid Homoploid means "at the same ploidy level", i.e. having the same number of
homologous chromosomes. For example, homoploid
hybridization is hybridization where the offspring have the same ploidy level as the two parental species. This contrasts with a common situation in plants where chromosome doubling accompanies or occurs soon after hybridization. Similarly, homoploid speciation contrasts with
polyploid speciation.
Zygoidy and azygoidy Zygoidy is the state in which the chromosomes are paired and can undergo meiosis. The zygoid state of a species may be diploid or polyploid. In the azygoid state the chromosomes are unpaired. It may be the natural state of some asexual species or may occur after meiosis. In diploid organisms the azygoid state is monoploid. (See below for dihaploidy.) ==Special cases==