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Ploidy () is the number of complete sets of
chromosome A chromosome is a long DNA molecule with part or all of the genome, genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones which, aided by Chaperone (protein), chaperone proteins, bind to and ...

chromosome
s in a cell, and hence the number of possible
allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is one of two, or more, forms of a given gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance ...
s for
autosomal An autosome is any chromosome that is not a sex chromosome (an allosome). The members of an autosome pair in a diploid Ploidy () is the number of complete sets of chromosome A chromosome is a long DNA molecule with part or all of the ...
and
pseudoautosomal
pseudoautosomal
genes In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechani ...
. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively, in each homologous chromosome pair, which chromosomes naturally exist as.
Somatic cell A somatic cell (from Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek (, ...
s, tissues, and
individual An individual is that which exists as a distinct entity An entity is something that exists as itself, as a subject or as an object, actually or potentially, concretely or abstractly, physically or not. It need not be of material existence. In ...

individual
organisms can be described according to the number of sets of chromosomes present (the "ploidy level"): monoploid (1 set), diploid (2 sets), triploid (3 sets), tetraploid (4 sets), pentaploid (5 sets), hexaploid (6 sets), heptaploid or septaploid (7 sets), etc. The generic term
polyploid Polyploidy is a condition in which the biological cell, cells of an organism have more than two paired (Homologous chromosome, homologous) sets of chromosomes. Most species whose cells have Cell nucleus, nuclei (eukaryotes) are diploid, meaning t ...
is often used to describe cells with three or more chromosome sets. Virtually all sexually reproducing organisms are made up of somatic cells that are diploid or greater, but ploidy level may vary widely between different organisms, between different tissues within the same organism, and at different stages in an organism's life cycle. Half of all known plant genera contain polyploid species, and about two-thirds of all grasses are polyploid. Many animals are uniformly diploid, though polyploidy is common in invertebrates, reptiles, and amphibians. In some species, ploidy varies between individuals of the same species (as in the
social insect Eusociality (from Greek εὖ ''eu'' "good" and social), the highest level of organization of sociality, is defined by the following characteristics: cooperative brood care (including care of offspring from other individuals), overlapping gene ...
s), and in others entire tissues and organ systems may be polyploid despite the rest of the body being diploid (as in the mammalian
liver The liver is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's t ...

liver
). For many organisms, especially plants and fungi, changes in ploidy level between generations are major drivers of
speciation Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within ...

speciation
. In mammals and birds, ploidy changes are typically fatal. There is, however, evidence of polyploidy in organisms now considered to be diploid, suggesting that polyploidy has contributed to evolutionary diversification in plants and animals through successive rounds of polyploidization and rediploidization. Humans are diploid organisms, normally carrying two complete sets of chromosomes in their somatic cells: two copies of paternal and maternal chromosomes, respectively, in each of the 23 homologous pairs of chromosomes that humans normally have. This results in two homologous pairs within each of the 23 homologous pairs, providing a full complement of 46 chromosomes. This total number of individual chromosomes (counting all complete sets) is called the chromosome number or chromosome complement. The number of chromosomes found in a single complete set of chromosomes is called the monoploid number (''x''). The haploid number (''n'') refers to the total number of chromosomes found in a
gamete A gamete ( /ˈɡæmiːt/; from Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply ...
(a
sperm Sperm is the male reproductive Cell (biology), cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as ...

sperm
or
egg An egg is the organic vessel containing the in which an develops until it can survive on its own, at which point the animal hatches. An egg results from of an . Most s, (excluding s), and lay eggs, although some, such as s, do not. eg ...

egg
cell produced by
meiosis Meiosis (; , because it is a reductional division) is a special type of of in organisms used to produce the , such as or . It involves two rounds of division that ultimately result in four cells with only one copy of each (). Additionall ...

meiosis
in preparation for sexual reproduction). Under normal conditions, the haploid number is exactly half the total number of chromosomes present in the organism's somatic cells, with one paternal and maternal copy in each chromosome pair. For diploid organisms, the monoploid number and haploid number are equal; in humans, both are equal to 23. When a human
germ cell A germ cell is any biological cell The cell (from Latin ''cella'', meaning "small room") is the basic structural, functional, and biological unit of all known organisms. Cells are the smallest units of life, and hence are often referred to a ...
undergoes meiosis, the diploid 46 chromosome complement is split in half to form haploid gametes. After fusion of a male and a female gamete (each containing 1 set of 23 chromosomes) during
fertilization Fertilisation or fertilization (see American and British English spelling differences#-ise.2C -ize .28-isation.2C -ization.29, spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes ...

fertilization
, the resulting
zygote A zygote (, ) is a eukaryotic Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are ...

zygote
again has the full complement of 46 chromosomes: 2 sets of 23 chromosomes.
Euploidy and aneuploidy Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Somatic cells, Tissue (biology), tissues, and I ...
describe having a number of chromosomes that is an exact multiple of the number of chromosomes in a normal gamete; and having any other number, respectively. For example, a person with
Turner syndrome Turner syndrome (TS), also known as 45,X, or 45,X0, is a genetic condition A genetic disorder is a health problem caused by one or more abnormalities in the genome. It can be caused by a mutation in a single gene In biology, a gene ...
may be missing one sex chromosome (X or Y), resulting in a (45,X) karyotype instead of the usual (46,XX) or (46,XY). This is a type of aneuploidy and cells from the person may be said to be aneuploid with a (diploid) chromosome complement of 45.


Etymology

The term ''ploidy'' is a
back-formation In etymology Etymology ()The New Oxford Dictionary of English ''The'' () is a grammatical article Article often refers to: * Article (grammar) An article is any member of a class of dedicated words that are used with noun phrases to ...
from ''haploidy'' and ''diploidy''. "Ploid" is a combination of Ancient Greek -πλόος (-plóos, “-fold”) and -ειδής (-''eidḗs''), from εἶδος (''eîdos'', "form, likeness"). The principal meaning of the
Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
word ᾰ̔πλόος (haplóos) is "single", from ἁ- (ha-, “one, same”). διπλόος (''diplóos'') means "duplex" or "two-fold". Diploid therefore means "duplex-shaped" (compare "humanoid", "human-shaped"). Polish botanist
Eduard Strasburger Eduard Adolf Strasburger (1 February 1844 – 18 May 1912) was a Polish- German professor and one of the most famous botanist Botany, also called , plant biology or phytology, is the science Science (from the Latin word ''scientia'', ...
coined the terms ''haploid'' and ''diploid'' in 1905. Some authors suggest that Strasburger based the terms on
August Weismann Prof August Friedrich Leopold Weismann FRS (For), HonFRSE, LLD (17 January 18345 November 1914) was a German evolutionary biologist. Ernst Mayr Ernst Walter Mayr (; 5 July 1904 – 3 February 2005) was one of the 20th century's leading ...

August Weismann
's conception of the id (or
germ plasm upAugust Weismann proposed the germ plasm theory in the 19th century, before the foundation of modern genetics. Germ plasm () is a biological concept developed in the 19th century by the German biologist August Weismann. It states that heritable ...
), hence haplo-''id'' and diplo-''id''. The two terms were brought into the English language from German through
William Henry Lang William Henry Lang FRS FRS may also refer to: Government and politics * Facility Registry System, a centrally managed Environmental Protection Agency database that identifies places of environmental interest in the United States * Family Resourc ...
's 1908 translation of a 1906 textbook by Strasburger and colleagues.


Types of ploidy


Haploid and monoploid

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 A gamete ( /ˈɡæmiːt/; from Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply ...
. 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 Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...

eukaryotic
cell is only said to be haploid if it has a single set of
chromosome A chromosome is a long DNA molecule with part or all of the genome, genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones which, aided by Chaperone (protein), chaperone proteins, bind to and ...

chromosome
s, 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.
Gamete A gamete ( /ˈɡæmiːt/; from Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply ...
s (
sperm Sperm is the male reproductive Cell (biology), cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as ...

sperm
and
ova , abbreviated as OVA and sometimes as OAV (original animation video), are Japanese animated films and series made specially for release in home video Home video is prerecorded media sold or Video rental shop, rented for home viewing. The ter ...
) are haploid cells. The haploid gametes produced by most organisms combine to form a
zygote A zygote (, ) is a eukaryotic Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are ...

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 Meiosis (; , because it is a reductional division) is a special type of of in organisms used to produce the , such as or . It involves two rounds of division that ultimately result in four cells with only one copy of each (). Additionall ...

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
plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel ...

plant
s and many
fungi A fungus (plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full ...

fungi
and
alga Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Co ...

alga
e switch between a haploid and a diploid state, with one of the stages emphasized over the other. This is called
alternation of generations Alternation of generations (also known as metagenesis or heterogenesis) is the type of life cycle Life cycle, life-cycle, or lifecycle may refer to: Science and academia *Biological life cycle, the sequence of life stages that an organism unde ...

alternation of generations
. Most fungi and algae are haploid during the principal stage of their life cycle, as are some primitive plants like
moss Mosses are small, non-vascular flower A flower, sometimes known as a bloom or blossom Cherry blossoms in Paris in full bloom. In botany, blossoms are the flowers of stone fruit fruit tree, trees (genus ''Prunus'') and of some other plant ...

moss
es. 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 Bees are flying insects closely related to wasps and ants, known for their role in pollination and, in the case of the best-known bee species, the western honey bee, for producing honey. Bees are a monophyly, monophyletic lineage within the ...

bees
,
wasps A wasp is any insect of the narrow-waisted suborder Apocrita of the order Hymenoptera which is neither a bee nor an ant; this excludes the broad-waisted sawflies (Symphyta), which look somewhat like wasps, but are in a separate suborder. The ...

wasps
, and
ants Ants are eusocial Eusociality (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is ...

ants
are haploid organisms because they develop from unfertilized, haploid eggs, while females (workers and queens) are diploid, making their system
haplodiploid Haplodiploidy is a sex-determination system A sex-determination system is a biological system that determines the development of sexual characteristics in an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισ ...
. 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 Common wheat (''Triticum aestivum''), also known as bread wheat, is a cultivated wheat Wheat is a grass widely cultivated for its seed, a cereal grain which is a worldwide staple food. The many species of wheat together make up the genus ...
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

Diploid cells have two homologous copies of each
chromosome A chromosome is a long DNA molecule with part or all of the genome, genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones which, aided by Chaperone (protein), chaperone proteins, bind to and ...

chromosome
, usually one from the
mother A mother is the female Female (symbol: ♀) is the sex of an organism, or a part of an organism, that produces non-mobile ovum, ova (egg cells). Barring rare medical conditions, most female mammals, including female humans, have two X chro ...

mother
and one from the
father A father is the male Male (symbol: ♂) is the sex of an organism that produces the gamete (sex cell) known as sperm, which fuses with the larger female gamete, or ovum, in the process of fertilization. A male organism cannot sexual reprod ...

father
. All or nearly all mammals are diploid organisms. The suspected tetraploid (possessing four-chromosome sets) plains viscacha rat (''
Tympanoctomys barrerae
Tympanoctomys barrerae
'') and golden viscacha rat (''
Pipanacoctomys aureus The golden viscacha rat or golden vizcacha rat (''Pipanacoctomys aureus'') is the single species of the genus ''Pipanacoctomys'' of the rodent family In human society, family (from la, familia) is a group of people related either by consa ...
'') have been regarded as the only known exceptions (as of 2004). However, some genetic studies have rejected any
polyploid Polyploidy is a condition in which the biological cell, cells of an organism have more than two paired (Homologous chromosome, homologous) sets of chromosomes. Most species whose cells have Cell nucleus, nuclei (eukaryotes) are diploid, meaning t ...
ism 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
polyploid Polyploidy is a condition in which the biological cell, cells of an organism have more than two paired (Homologous chromosome, homologous) sets of chromosomes. Most species whose cells have Cell nucleus, nuclei (eukaryotes) are diploid, meaning t ...
y.
Human Humans (''Homo sapiens'') are the most abundant and widespread species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A speci ...

Human
diploid cells have 46 chromosomes (the
somatic Somatic may refer to: * Somatic (biology), referring to the cells of the body in contrast to the germ line cells ** Somatic cell, a non-gametic cell in a multicellular organism * Somatic nervous system, the portion of the vertebrate nervous syste ...
number, ''2n'') and human haploid
gametes A gamete ( /ˈɡæmiːt/; from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the foll ...
(egg and sperm) have 23 chromosomes (''n'').
Retrovirus A retrovirus is a type of virus A virus is a submicroscopic infectious agent In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical ...

Retrovirus
es 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 The human T-lymphotropic virus, human T-cell lymphotropic virus, or human T-cell leukemia-lymphoma virus (HTLV) family of viruses are a group of human retroviruses that are known to cause a type of cancer called adult T-cell leukemia/lymphoma and ...
, and
HIV The human immunodeficiency viruses (HIV) are two species of ''Lentivirus ''Lentivirus'' is a genus Genus (plural genera) is a taxonomic rank Taxonomy (general) is the practice and science of classification of things or concepts, inc ...

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 or septaploid (7 sets), octoploid (8 sets), nonaploid (9 sets), decaploid (10 sets), undecaploid (11 sets), dodecaploid (12 sets), tridecaploid (13 sets), tetradecaploid (14 sets), etc. 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"). Polytene chromosomes of plants and fruit flies can be 1024-ploid. Ploidy of systems such as the
salivary gland The salivary glands in mammal Mammals (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'' ...
,
elaiosome '' seeds bearing orange elaiosomes Image:TrilliumRecurvatumSeeds.jpg">''Trillium recurvatum'' seeds Elaiosomes ( grc, ἔλαιον ''élaion'' "oil" + ''sóma'' "body") are fleshy structures that are attached to the seeds of many plant species. ...
,
endosperm The endosperm is a tissue produced inside the seed A seed is an embryonic ''Embryonic'' is the twelfth studio album by experimental rock band the Flaming Lips released on October 13, 2009, on Warner Bros. Records, Warner Bros. The band's fir ...
, and
trophoblast Trophoblasts (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approx ...
can exceed this, up to 1048576-ploid in the silk glands of the commercial silkworm ''
Bombyx mori ''Bombyx mori'', the domestic silk moth, is an insect from the moth Moths are a paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and all descendants of that ancestor excluding a ...

Bombyx mori
''. The chromosome sets may be from the same species or from closely related species. In the latter case, these are known as allopolyploids (or amphidiploids, which are allopolyploids that behave as if they were normal diploids). Allopolyploids are formed from the hybridization of two separate species. In plants, this probably most often occurs from the pairing of meiotically unreduced
gametes A gamete ( /ˈɡæmiːt/; from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the foll ...
, and not by diploid–diploid hybridization followed by chromosome doubling. 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 cell A somatic cell (from Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek (, ...
s are polyploid due to a process called
endoreduplication Endoreduplication (also referred to as endoreplication or endocycling) is replication of the nuclear genome In the fields of molecular biology Molecular biology is the branch of biology Biology is the natural science that stud ...
, where duplication of the
genome In the fields of molecular biology Molecular biology is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, M ...

genome
occurs without
mitosis In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical proce ...

mitosis
(cell division). The extreme in polyploidy occurs in the
fern A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plant Vascular plants (from Latin ''vasculum'': duct), also known as Tracheophyta (the tracheophytes , from Greek τραχεῖα ἀρτηρία ''trācheia art ...

fern
genus ''
Ophioglossum ''Ophioglossum'', the adder's-tongue ferns, is a genus of about 50 species of ferns in the family Ophioglossaceae. The name ''Ophioglossum'' comes from the Greek language, Greek meaning "snake-tongue".
'', 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
haploidisationHaploidisation is the process of halving the chromosomal content of a cell, creating a haploid Ploidy () is the number of complete sets of chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organ ...
.


In bacteria and archaea

Polyploid Polyploidy is a condition in which the biological cell, cells of an organism have more than two paired (Homologous chromosome, homologous) sets of chromosomes. Most species whose cells have Cell nucleus, nuclei (eukaryotes) are diploid, meaning t ...
y is a characteristic of the bacterium ''
Deinococcus radiodurans ''Deinococcus radiodurans'' is an extremophilic bacterium Bacteria (; common noun bacteria, singular bacterium) are a type of biological cell The cell (from Latin ''cella'', meaning "small room") is the basic structural, functional, ...

Deinococcus radiodurans
'' and of the
archaeon Archaea ( ; singular archaeon ) constitute a domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a pa ...

archaeon
'' Halobacterium salinarum''. These two species are highly resistant to
ionizing radiation Ionizing radiation (or ionising radiation), including nuclear radiation, consists of s or s that have sufficient to s or s by detaching s from them. The particles generally travel at a speed that is greater than 1% of , and the electromagnetic w ...
and
desiccation Desiccation (from Latin de- "thoroughly" + siccare "to dry") is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic (attracts and holds water) substance that induces or sustains such a state in its lo ...
, conditions that induce
DNA Deoxyribonucleic acid (; DNA) is a molecule A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically Electricity is the set of physical ...

DNA
double-strand breaks. This resistance appears to be due to efficient
homologous recombination Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids (usually DNA as in Cell (biology), cellular organi ...

homologous recombination
al repair.


Variable or indefinite ploidy

Depending on growth conditions,
prokaryote A prokaryote () is a single-celled organism A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual conti ...
s such as
bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

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
haploidisationHaploidisation is the process of halving the chromosomal content of a cell, creating a haploid Ploidy () is the number of complete sets of chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organ ...
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#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
''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
karyotype A karyotype is a preparation of the complete set of metaphase Metaphase () is a stage of mitosis In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies ...

karyotype
s would consequentially be a multiple of the haploid number, which in humans is 23.
Aneuploidy Aneuploidy is the presence of an abnormal number of chromosome A chromosome is a long DNA molecule with part or all of the genome, genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones whic ...
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 Down syndrome or Down's syndrome, also known as trisomy 21, is a genetic disorder A genetic disorder is a health problem caused by one or more abnormalities in the genome In the fields of molecular biology and genetics Gen ...
, where affected individuals have three copies of chromosome 21) or missing a chromosome (as in
Turner syndrome Turner syndrome (TS), also known as 45,X, or 45,X0, is a genetic condition A genetic disorder is a health problem caused by one or more abnormalities in the genome. It can be caused by a mutation in a single gene In biology, a gene ...
, where affected individuals have only one sex chromosome). Aneuploid
karyotype A karyotype is a preparation of the complete set of metaphase Metaphase () is a stage of mitosis In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies ...

karyotype
s are given names with the suffix ''-somy'' (rather than ''-ploidy'', used for euploid karyotypes), such as
trisomy A trisomy is a type of polysomy in which there are three instances of a particular chromosome A chromosome is a long DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling mic ...
and
monosomy Monosomy is a form of aneuploidy Aneuploidy is the presence of an abnormal number of chromosomes in a cell (biology), cell, for example a human cell having 45 or 47 chromosomes instead of the usual 46. It does not include a difference of one or mor ...
.


Homoploid

Homoploid means "at the same ploidy level", i.e. having the same number of
homologous chromosome A couple of homologous chromosomes, or homologs, are a set of one maternal and one paternal chromosome A chromosome is a long DNA The structure of part of a DNA double helix Deoxyribonucleic acid (; DNA) is a molecule File:Pen ...
s. 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.Cosín, Darío J. Díaz, Marta Novo, and Rosa Fernández.
Reproduction of Earthworms: Sexual Selection and Parthenogenesis.
In Biology of Earthworms, edited by Ayten Karaca, 24:69–86. Berlin, Heidelberg: Springer Berlin Heidelberg, 2011.
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


More than one nucleus per cell

In the strictest sense, ploidy refers to the number of sets of chromosomes in a single
nucleus ''Nucleus'' (plural nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...
rather than in the cell as a whole. Because in most situations there is only one nucleus per cell, it is commonplace to speak of the ploidy of a cell, but in cases in which there is more than one nucleus per cell, more specific definitions are required when ploidy is discussed. Authors may at times report the total combined ploidy of all nuclei present within the cell membrane of a
syncytium A syncytium or symplasm (; plural syncytia; from Greek: σύν ''syn'' "together" and κύτος ''kytos'' "box, i.e. cell") is a multinucleate cell which can result from multiple cell fusions of uninuclear cells (i.e., cells with a single nucleu ...
,Encyclopedia of the Life Sciences (2002) "Polyploidy" Francesco D'Amato and Mauro Durante though usually the ploidy of each nucleus is described individually. For example, a fungal
dikaryon The dikaryon is a nuclear feature which is unique to certain fungi A fungus (plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. ...
with two separate haploid nuclei is distinguished from a diploid cell in which the chromosomes share a nucleus and can be shuffled together.


Ancestral ploidy levels

It is possible on rare occasions for ploidy to increase in the
germline In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms ...
, which can result in
polyploid Polyploidy is a condition in which the biological cell, cells of an organism have more than two paired (Homologous chromosome, homologous) sets of chromosomes. Most species whose cells have Cell nucleus, nuclei (eukaryotes) are diploid, meaning t ...
offspring and ultimately polyploid species. This is an important evolutionary mechanism in both plants and animals and is known as a primary driver of
speciation Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within ...

speciation
. As a result, it may become desirable to distinguish between the ploidy of a species or variety as it presently breeds and that of an ancestor. The number of chromosomes in the ancestral (non-homologous) set is called the monoploid number (''x''), and is distinct from the haploid number (''n'') in the organism as it now reproduces.
Common wheat Common wheat (''Triticum aestivum''), also known as bread wheat, is a cultivated wheat Wheat is a grass widely cultivated for its seed, a cereal grain which is a worldwide staple food. The many species of wheat together make up the genus ...
(''Triticum aestivum'') is an organism in which ''x'' and ''n'' differ. Each plant has a total of six sets of chromosomes (with two sets likely having been obtained from each of three different diploid species that are its distant ancestors). The somatic cells are hexaploid, 2''n'' = 6''x'' = 42 (where the monoploid number ''x'' = 7 and the haploid number ''n'' = 21). The gametes are haploid for their own species, but triploid, with three sets of chromosomes, by comparison to a probable evolutionary ancestor,
einkorn wheat Einkorn wheat (from German ''Einkorn'', literally "single grain") can refer either to the wild species of wheat Wheat is a grass widely cultivated for its seed, a cereal grain which is a worldwide staple food. The many species of wheat tog ...
.
Tetraploidy Polyploidy is a condition in which the cells of an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. ...
(four sets of chromosomes, 2''n'' = 4''x'') is common in many
plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel ...

plant
species, and also occurs in
amphibian Amphibians are ectothermic, tetrapod vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial animal, ter ...
s,
reptile Reptiles, as most commonly defined, are the animals in the class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or ...

reptile
s, and
insect Insects (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in ...

insect
s. For example, species of ''
Xenopus ''Xenopus'' () (Gk., ξενος, ''xenos''=strange, πους, ''pous''=foot, commonly known as the clawed frog) is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In bi ...

Xenopus
'' (African toads) form a ploidy series, featuring diploid ('' X. tropicalis'', 2n=20), tetraploid ('''', 4n=36), octaploid ('' X. wittei'', 8n=72), and dodecaploid ('' X. ruwenzoriensis'', 12n=108) species. Over evolutionary time scales in which
chromosomal polymorphism In genetics Genetics is a branch of biology concerned with the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) Though heredity had been observed for millennia, Gregor Mendel, Moravia, Moravian scientist an ...
s accumulate, these changes become less apparent by
karyotype A karyotype is a preparation of the complete set of metaphase Metaphase () is a stage of mitosis In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies ...

karyotype
– for example, humans are generally regarded as diploid, but the
2R hypothesisThe 2R hypothesis or Ohno's hypothesis, first proposed by Susumu Ohno in 1970,Ohno, Susumu (1970). ''Evolution by Gene Duplication.'' London: Allen and Unwin, . is a hypothesis A hypothesis (plural hypotheses) is a proposed explanation for a ...
has confirmed two rounds of whole genome duplication in early vertebrate ancestors.


Haplodiploidy

Ploidy can also vary between individuals of the same species or at different stages of the
life cycle Life cycle, life-cycle, or lifecycle may refer to: Science and academia *Biological life cycle, the sequence of life stages that an organism undergoes from birth to reproduction ending with the production of the offspring *Life-cycle hypothesis, ...
. In some insects it differs by
caste Caste is a form of social stratification Social stratification refers to a society's categorization Categorization is the ability and activity to recognize shared features or similarities between the elements of the experience of the ...
. In humans, only the gametes are haploid, but in many of the
social insect Eusociality (from Greek εὖ ''eu'' "good" and social), the highest level of organization of sociality, is defined by the following characteristics: cooperative brood care (including care of offspring from other individuals), overlapping gene ...
s, including
ant Ants are eusocial Eusociality (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population i ...

ant
s,
bee Bees are flying insect Insects (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', m ...

bee
s, and
termite Termites are eusocial Eusociality (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its populati ...

termite
s, certain individuals develop from unfertilized eggs, making them haploid for their entire lives, even as adults. In the Australian bulldog ant, '' Myrmecia pilosula'', a
haplodiploid Haplodiploidy is a sex-determination system A sex-determination system is a biological system that determines the development of sexual characteristics in an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισ ...
species, haploid individuals of this species have a single chromosome and diploid individuals have two chromosomes. In ''
Entamoeba ''Entamoeba'' is a genus Genus (plural genera) is a taxonomic rank Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer to ...
'', the ploidy level varies from 4''n'' to 40''n'' in a single population.
Alternation of generations Alternation of generations (also known as metagenesis or heterogenesis) is the type of life cycle Life cycle, life-cycle, or lifecycle may refer to: Science and academia *Biological life cycle, the sequence of life stages that an organism unde ...

Alternation of generations
occurs in most plants, with individuals "alternating" ploidy level between different stages of their sexual life cycle.


Tissue-specific polyploidy

In large multicellular organisms, variations in ploidy level between different tissues, organs, or cell lineages are common. Because the chromosome number is generally reduced only by the specialized process of meiosis, the somatic cells of the body inherit and maintain the chromosome number of the zygote by mitosis. However, in many situations somatic cells double their copy number by means of
endoreduplication Endoreduplication (also referred to as endoreplication or endocycling) is replication of the nuclear genome In the fields of molecular biology Molecular biology is the branch of biology Biology is the natural science that stud ...
as an aspect of
cellular differentiation Cellular differentiation is the process in which a cell changes from one cell type A cell type is a classification used to distinguish between morphologically or phenotypically distinct cell forms within a species In biology, a sp ...
. For example, the hearts of two-year-old human children contain 85% diploid and 15% tetraploid nuclei, but by 12 years of age the proportions become approximately equal, and adults examined contained 27% diploid, 71% tetraploid and 2% octaploid nuclei.


Adaptive and ecological significance of variation in ploidy

There is continued study and debate regarding the fitness advantages or disadvantages conferred by different ploidy levels. A study comparing the
karyotypes Karyotyping is the process by which photographs of chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones which, aided ...
of endangered or invasive plants with those of their relatives found that being polyploid as opposed to diploid is associated with a 14% lower risk of being endangered, and a 20% greater chance of being invasive. Polyploidy may be associated with increased vigor and adaptability. Some studies suggest that selection is more likely to favor diploidy in host species and haploidy in parasite species. When a germ cell with an uneven number of chromosomes undergoes meiosis, the chromosomes cannot be evenly divided between the daughter cells, resulting in
aneuploid Aneuploidy is the presence of an abnormal number of chromosome A chromosome is a long DNA molecule with part or all of the genome, genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones whic ...
gametes. Triploid organisms, for instance, are usually sterile. Because of this, triploidy is commonly exploited in agriculture to produce seedless fruit such as bananas and watermelons. If the fertilization of human gametes results in three sets of chromosomes, the condition is called
triploid syndrome Triploid syndrome, also called triploidy, is a chromosomal disorder A chromosome abnormality, chromosomal anomaly, chromosomal aberration, chromosomal mutation, or chromosomal disorder, is a missing, extra, or irregular portion of chromosomal DNA. ...
. In
unicellular organism A unicellular organism, also known as a single-celled organism, is an organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemica ...
s the ploidy nutrient limitation hypothesis suggests that nutrient limitation should encourage haploidy in preference to higher ploidies. This hypothesis is due to the higher surface-to-volume ratio of haploids, which eases nutrient uptake, thereby increasing the internal nutrient-to-demand ratio. Mable 2001 finds ''
Saccharomyces cerevisiae ''Saccharomyces cerevisiae'' () is a species of yeast Yeasts are eukaryotic Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are ...

Saccharomyces cerevisiae
'' to be somewhat inconsistent with this hypothesis however, as haploid growth is faster than diploid under high nutrient conditions. The NLH is also tested in haploid, diploid, and polyploid fungi by Gerstein et al 2017. This result is also more complex: On the one hand, under
phosphorus Phosphorus is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical el ...

phosphorus
and other nutrient limitation, lower ploidy is selected as expected. However under normal nutrient levels or under limitation of only
nitrogen Nitrogen is the chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science ...

nitrogen
, higher ploidy was selected. Thus the NLH and more generally, the idea that polyploidy is selected by harsher conditions is cast into doubt by these results. Older WGDs have also been investigated. Only as recently as 2015 was the ancient
whole genome duplication Whole may refer to: Music * Whole note A whole note (American) or semibreve (British) in musical notation is a single note equivalent to or lasting as long as two half-notes or four quarter-notes. Description The whole note or semibreve has ...
in
Baker's yeast Baker's yeast is the common name for the strains of yeast Yeasts are eukaryotic Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, ph ...
proven to be
allopolyploid Polyploidy is a condition in which the cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse ...
, by Marcet-Houben and Gabaldón 2015. It still remains to be explained why there are not more polyploid events in fungi, and the place of neopolyploidy and mesopolyploidy in fungal history.


Glossary of ploidy numbers

The common
potato The potato is a starch#Food, starchy tuber of the plant ''Solanum tuberosum'' and is a root vegetable native to the Americas. The plant is a perennial plant, perennial in the nightshade family Solanaceae. Wild potato species can be found thro ...

potato
(''Solanum tuberosum'') is an example of a tetraploid organism, carrying four sets of chromosomes. During sexual reproduction, each potato plant inherits two sets of 12 chromosomes from the pollen parent, and two sets of 12 chromosomes from the ovule parent. The four sets combined provide a full complement of 48 chromosomes. The haploid number (half of 48) is 24. The monoploid number equals the total chromosome number divided by the ploidy level of the somatic cells: 48 chromosomes in total divided by a ploidy level of 4 equals a monoploid number of 12. Hence, the monoploid number (12) and haploid number (24) are distinct in this example. However, commercial potato crops (as well as many other crop plants) are commonly propagated vegetatively (by asexual reproduction through mitosis), in which case new individuals are produced from a single parent, without the involvement of gametes and fertilization, and all the offspring are genetically identical to each other and to the parent, including in chromosome number. The parents of these vegetative clones may still be capable of producing haploid gametes in preparation for sexual reproduction, but these gametes are not used to create the vegetative offspring by this route.


Specific examples


Notes


References


Sources

*Griffiths, A. J. ''et al.'' 2000. ''An introduction to genetic analysis'', 7th ed. W. H. Freeman, New York


External links

Some eukaryotic genome-scale or
genome size Genome size is the total amount of DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A ...
databases and other sources which may list the ploidy levels of many organisms:
Animal genome size databaseFungal genome size database
of
Ensembl Genomes Ensembl Genomes is a scientific project to provide genome-scale data from non-vertebrate species. The project is run by the European Bioinformatics Institute, and was launched in 2009 using the Ensembl technology. The main objective of the Ensembl ...
*
Supporting Data Set
with information on ploidy level and number of chromosomes of several protists)
Chromosome number and ploidy mutations
YouTube tutorial video {{chromo Classical genetics Cytogenetics Genetics concepts