Ploidy
Encyclopedia
Ploidy is the number of sets of chromosomes in a biological cell.
Human sex cells (sperm
and egg
) have one complete set of chromosomes from the male or female parent. Sex cells, also called gamete
s, combine to produce somatic cells. Somatic cells, therefore, have twice as many chromosomes. The haploid number (n) is the number of chromosomes in a gamete. A somatic cell has twice that many chromosomes (2n).
Humans are diploid. A human somatic cell contains 46 chromosomes: 2 complete haploid sets, which make up 23 homologous chromosome
pairs. However, many organisms have more than two sets of homologous chromosomes and are called polyploid
.
The number of chromosomes in a single (non-homologous) set is called the monoploid number (x), and is different from the haploid number (n). Both numbers n, and x, apply to every cell of a given organism. For humans, x = n = 23, which is also written as 2n = 2x = 46. Bread wheat
is an organism where x and n differ. It has six sets of chromosomes, two sets from each of three different diploid species that are its distant ancestors. The somatic cells are hexaploid, with six sets of chromosomes, 2n = 6x = 42. The gametes are both haploid and triploid, with three sets of chromosomes. The monoploid number x = 7, and the haploid number n = 21.
Tetraploidy (four sets of chromosomes, 2n = 4x) is common in plant
s, and also occurs in amphibian
s, reptile
s, and insect
s.
The Australian bulldog ant, Myrmecia pilosula, a haplodiploid species, has n = x = 1, the lowest chromosome number theoretically possible. Haploid individuals of this species have a single chromosome, and diploid individuals have two chromosomes.
Euploidy is the state of a cell or organism having an integral multiple of the monoploid number, possibly excluding the sex-determining chromosomes. For example, a human cell has 46 chromosomes, which is an integer
multiple of the monoploid number, 23. A human with abnormal, but integral, multiples of this full set (e.g. 69 chromosomes) would also be considered as euploid. Aneuploidy
is the state of not having euploidy. In humans, examples include having a single extra chromosome (such as Down syndrome
), or missing a chromosome (such as Turner syndrome
). Aneuploid karyotype
s are given names with the suffix -somy (rather than -ploidy, used for euploid karyotypes), such as trisomy
and monosomy
.
from haploid and diploid. These two terms are from Greek
ἁπλόος haplóos "single" and διπλόος diplóos "double" combined with εἶδος eîdos "form" (compare idol from Latin īdōlum, that from Greek εἴδωλον eídōlon derived from εἶδος eîdos). The two haploid and diploid terms were borrowed from German through William Henry Lang
's 1908 translation of a 1894 textbook by Eduard Strasburger
and colleagues. Strasburger used diploid to refer to an organism with twice the number of chromosomes of a haploid organism, hence "double" and "single".
of an individual, and this is distinct from the monoploid number (x), which is the number of unique chromosomes in a single complete set. Gametes (sperm
, and ova
) are haploid cells. The haploid gametes produced by (most) diploid organisms are monoploid, and these can combine to form a diploid zygote
. For example, most animals are diploid and produce monoploid gametes.
During meiosis
, sex cell precursors have their number of chromosomes halved by randomly "choosing" one homologue, resulting in haploid gametes. Because homologous chromosomes usually differ genetically, gametes usually differ genetically from one another.
All plant
s and many fungi and algae switch between a haploid and a diploid state (which may be polyploid), 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 lifecycle.
Male
bees, wasps, and ants are haploid organisms because of the way they develop from unfertilized, haploid egg cells.
In humans, the monoploid number (x) equals the haploid number (n), x = n = 23, but, in some species (especially plants), these numbers differ. Common wheat
has six sets of chromosomes in the somatic
cells, derived from its three different ancestral species. The gametes of common wheat are considered to be haploid, since they contain half the genetic information of somatic cells, but are not monoploid, as they still contain three complete sets of chromosomes (n = 3x).
copies of each chromosome
, usually one from the mother
and one from the father
. Nearly all mammals are diploid organisms (the tetraploid viscacha rat
s Pipanacoctomys aureus and Tympanoctomys barrerae are the only known exceptions as of 2004), although all individuals have some small fraction of cells that display polyploidy
. Human diploid cells have 46 chromosomes and human haploid gametes (egg and sperm) have 23 chromosomes.
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
, and HIV
.
A laboratory procedure called haploidisation forces a normal cell to expel half of its chromosomal complement. In mammals this renders this cell chromosomally equal to sperm
or egg
. This was one of the procedures used by Japan
ese researchers to produce Kaguya, a fatherless mouse
.
Haploidisation sometimes occurs in plants when meiotically reduced cells (usually egg cells) develop by parthenogenesis
.
A rare genetic disorder that has occurred in a total of 7 recorded cases is Detrimental Haploidy Syndrome where the somatic cells of the human body are haploid after the first division of cells from fertilisation. As a result of this a human with this syndrome is unfortunately prone to other diseases and unable to reproduce.
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
s 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.
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.
Human sex cells (sperm
Sperm
The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
and egg
Ovum
An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
) have one complete set of chromosomes from the male or female parent. Sex cells, also called gamete
Gamete
A gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...
s, combine to produce somatic cells. Somatic cells, therefore, have twice as many chromosomes. The haploid number (n) is the number of chromosomes in a gamete. A somatic cell has twice that many chromosomes (2n).
Humans are diploid. A human somatic cell contains 46 chromosomes: 2 complete haploid sets, which make up 23 homologous chromosome
Homologous chromosome
Homologous chromosomes are chromosome pairs of approximately the same length, centromere position, and staining pattern, with genes for the same characteristics at corresponding loci. One homologous chromosome is inherited from the organism's mother; the other from the organism's father...
pairs. However, many organisms have more than two sets of homologous chromosomes and are called polyploid
Polyploidy
Polyploid is a term used to describe cells and organisms containing more than two paired sets of chromosomes. Most eukaryotic species are diploid, meaning they have two sets of chromosomes — one set inherited from each parent. However polyploidy is found in some organisms and is especially common...
.
The number of chromosomes in a single (non-homologous) set is called the monoploid number (x), and is different from the haploid number (n). Both numbers n, and x, apply to every cell of a given organism. For humans, x = n = 23, which is also written as 2n = 2x = 46. Bread wheat
Common wheat
Common wheat, Triticum aestivum, is a cultivated wheat species.-Nomenclature and taxonomy of the variety and its cultivars:-Evolution:...
is an organism where x and n differ. It has six sets of chromosomes, two sets from each of three different diploid species that are its distant ancestors. The somatic cells are hexaploid, with six sets of chromosomes, 2n = 6x = 42. The gametes are both haploid and triploid, with three sets of chromosomes. The monoploid number x = 7, and the haploid number n = 21.
Tetraploidy (four sets of chromosomes, 2n = 4x) is common in plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
s, and also occurs in amphibian
Amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...
s, reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
s, and insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
s.
The Australian bulldog ant, Myrmecia pilosula, a haplodiploid species, has n = x = 1, the lowest chromosome number theoretically possible. Haploid individuals of this species have a single chromosome, and diploid individuals have two chromosomes.
Euploidy is the state of a cell or organism having an integral multiple of the monoploid number, possibly excluding the sex-determining chromosomes. For example, a human cell has 46 chromosomes, which is an integer
Integer
The integers are formed by the natural numbers together with the negatives of the non-zero natural numbers .They are known as Positive and Negative Integers respectively...
multiple of the monoploid number, 23. A human with abnormal, but integral, multiples of this full set (e.g. 69 chromosomes) would also be considered as euploid. Aneuploidy
Aneuploidy
Aneuploidy is an abnormal number of chromosomes, and is a type of chromosome abnormality. An extra or missing chromosome is a common cause of genetic disorders . Some cancer cells also have abnormal numbers of chromosomes. Aneuploidy occurs during cell division when the chromosomes do not separate...
is the state of not having euploidy. In humans, examples include having a single extra chromosome (such as Down syndrome
Down syndrome
Down syndrome, or Down's syndrome, trisomy 21, is a chromosomal condition caused by the presence of all or part of an extra 21st chromosome. It is named after John Langdon Down, the British physician who described the syndrome in 1866. The condition was clinically described earlier in the 19th...
), or missing a chromosome (such as Turner syndrome
Turner syndrome
Turner syndrome or Ullrich-Turner syndrome encompasses several conditions in human females, of which monosomy X is most common. It is a chromosomal abnormality in which all or part of one of the sex chromosomes is absent...
). Aneuploid karyotype
Karyotype
A karyotype is the number and appearance of chromosomes in the nucleus of an eukaryotic cell. The term is also used for the complete set of chromosomes in a species, or an individual organism.p28...
s are given names with the suffix -somy (rather than -ploidy, used for euploid karyotypes), such as trisomy
Trisomy
A trisomy is a type of polysomy in which there are three copies, instead of the normal two, of a particular chromosome. A trisomy is a type of aneuploidy .-Description and causes:...
and monosomy
Monosomy
Monosomy is a form of aneuploidy with the presence of only one chromosome from a pair. Partial monosomy occurs when only a portion of the chromosome has one copy, while the rest has two copies.-Human monosomy:...
.
Etymology
The term ploidy is a back-formationBack-formation
In etymology, back-formation is the process of creating a new lexeme, usually by removing actual or supposed affixes. The resulting neologism is called a back-formation, a term coined by James Murray in 1889...
from haploid and diploid. These two terms are from Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
ἁπλόος haplóos "single" and διπλόος diplóos "double" combined with εἶδος eîdos "form" (compare idol from Latin īdōlum, that from Greek εἴδωλον eídōlon derived from εἶδος eîdos). The two haploid and diploid terms were borrowed from German through William Henry Lang
William Henry Lang
William Henry Lang FRS was a British botanist. The son of Thomas Lang, a medical practitioner, Lang was educated at Dennistoun public school in Glasgow before being accepted into the University of Glasgow, where he graduated with a Bsc in botany and zoology in 1894...
's 1908 translation of a 1894 textbook by Eduard Strasburger
Eduard Strasburger
Eduard Adolf Strasburger was a German professor who was one of the most famous botanists of the 19th century....
and colleagues. Strasburger used diploid to refer to an organism with twice the number of chromosomes of a haploid organism, hence "double" and "single".
Haploid and monoploid
As stated above, the haploid number (n) is the number of chromosomes in a gameteGamete
A gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...
of an individual, and this is distinct from the monoploid number (x), which is the number of unique chromosomes in a single complete set. Gametes (sperm
Sperm
The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
, and ova
Ovum
An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
) are haploid cells. The haploid gametes produced by (most) diploid organisms are monoploid, and these can combine to form a diploid zygote
Zygote
A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...
. For example, most animals are diploid and produce monoploid gametes.
During meiosis
Meiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
, sex cell precursors have their number of chromosomes halved by randomly "choosing" one homologue, resulting in haploid gametes. Because homologous chromosomes usually differ genetically, gametes usually differ genetically from one another.
All plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
s and many fungi and algae switch between a haploid and a diploid state (which may be polyploid), with one of the stages emphasized over the other. This is called alternation of generations
Alternation of generations
Alternation of generations is a term primarily used in describing the life cycle of plants . A multicellular sporophyte, which is diploid with 2N paired chromosomes , alternates with a multicellular gametophyte, which is haploid with N unpaired chromosomes...
. Most fungi
Fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...
and algae are haploid during the principal stage of their lifecycle.
Male
Male
Male refers to the biological sex of an organism, or part of an organism, which produces small mobile gametes, called spermatozoa. Each spermatozoon can fuse with a larger female gamete or ovum, in the process of fertilization...
bees, wasps, and ants are haploid organisms because of the way they develop from unfertilized, haploid egg cells.
In humans, the monoploid number (x) equals the haploid number (n), x = n = 23, but, in some species (especially plants), these numbers differ. Common wheat
Common wheat
Common wheat, Triticum aestivum, is a cultivated wheat species.-Nomenclature and taxonomy of the variety and its cultivars:-Evolution:...
has six sets of chromosomes in the somatic
Somatic
The term somatic means 'of the body',, relating to the body. In medicine, somatic illness is bodily, not mental, illness. The term is often used in biology to refer to the cells of the body in contrast to the germ line cells which usually give rise to the gametes...
cells, derived from its three different ancestral species. The gametes of common wheat are considered to be haploid, since they contain half the genetic information of somatic cells, but are not monoploid, as they still contain three complete sets of chromosomes (n = 3x).
Diploid
Diploid (indicated by 2n = 2x) cells have two homologousHomologous chromosome
Homologous chromosomes are chromosome pairs of approximately the same length, centromere position, and staining pattern, with genes for the same characteristics at corresponding loci. One homologous chromosome is inherited from the organism's mother; the other from the organism's father...
copies of each chromosome
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
, usually one from the mother
Mother
A mother, mum, mom, momma, or mama is a woman who has raised a child, given birth to a child, and/or supplied the ovum that grew into a child. Because of the complexity and differences of a mother's social, cultural, and religious definitions and roles, it is challenging to specify a universally...
and one from the father
Father
A father, Pop, Dad, or Papa, is defined as a male parent of any type of offspring. The adjective "paternal" refers to father, parallel to "maternal" for mother...
. Nearly all mammals are diploid organisms (the tetraploid viscacha rat
Viscacha Rat
The viscacha rat , or mountain viscacha rat, is a species of rodent in the family Octodontidae. It is endemic to Chile and Argentina.It is the only living species within the genus Octomys...
s Pipanacoctomys aureus and Tympanoctomys barrerae are the only known exceptions as of 2004), although all individuals have some small fraction of cells that display polyploidy
Polyploidy
Polyploid is a term used to describe cells and organisms containing more than two paired sets of chromosomes. Most eukaryotic species are diploid, meaning they have two sets of chromosomes — one set inherited from each parent. However polyploidy is found in some organisms and is especially common...
. Human diploid cells have 46 chromosomes and human haploid gametes (egg and sperm) have 23 chromosomes.
Retrovirus
Retrovirus
A retrovirus is an RNA virus that is duplicated in a host cell using the reverse transcriptase enzyme to produce DNA from its RNA genome. The DNA is then incorporated into the host's genome by an integrase enzyme. The virus thereafter replicates as part of the host cell's DNA...
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 foamy virus
Human Foamy Virus is a retrovirus and specifically belongs to the genera Spumavirus. The spumaviruses are complex and significantly different from the other six genera or retroviruses in several ways...
, human T-lymphotropic virus
Human T-lymphotropic virus
Human T-cell Lymphotropic Virus Type 1 , also called the Adult T-cell lymphoma virus type 1, a virus that has been seriously implicated in several kinds of diseases including HTLV-I-associated myelopathy, Strongyloides stercoralis hyper-infection, and a virus cancer link for leukemia...
, and HIV
HIV
Human immunodeficiency virus is a lentivirus that causes acquired immunodeficiency syndrome , a condition in humans in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive...
.
Haploidisation
Haploidisation (haploidization) is the process of creating a haploid cell (usually from a diploid cell).A laboratory procedure called haploidisation forces a normal cell to expel half of its chromosomal complement. In mammals this renders this cell chromosomally equal to sperm
Spermatozoon
A spermatozoon is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote...
or egg
Ovum
An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
. This was one of the procedures used by Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
ese researchers to produce Kaguya, a fatherless mouse
Mouse
A mouse is a small mammal belonging to the order of rodents. The best known mouse species is the common house mouse . It is also a popular pet. In some places, certain kinds of field mice are also common. This rodent is eaten by large birds such as hawks and eagles...
.
Haploidisation sometimes occurs in plants when meiotically reduced cells (usually egg cells) develop by parthenogenesis
Parthenogenesis
Parthenogenesis is a form of asexual reproduction found in females, where growth and development of embryos occur without fertilization by a male...
.
A rare genetic disorder that has occurred in a total of 7 recorded cases is Detrimental Haploidy Syndrome where the somatic cells of the human body are haploid after the first division of cells from fertilisation. As a result of this a human with this syndrome is unfortunately prone to other diseases and unable to reproduce.
Polyploidy
Polyploidy is the state where all cells have multiple sets of chromosomes beyond the basic set, for example, in triploids 2n = 3x, in tetraploids 2n = 4x. 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, and not by diploid–diploid hybridization followed by chromosome doubling. The so-called Brassica triangleTriangle of U
The Triangle of U is a theory about the evolution and relationships between members of the plant genus Brassica. The theory states that the genomes of three ancestral species of Brassica combined to create three of the common contemporary vegetables and oilseed crop species...
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
Somatic cell
A somatic cell is any biological cell forming the body of an organism; that is, in a multicellular organism, any cell other than a gamete, germ cell, gametocyte or undifferentiated stem cell...
s are polyploid due to a process called endoreduplication
Endoreduplication
Endoreplication is replication of the nuclear genome in the absence of cell division, which leads to elevated nuclear gene content and polyploidy...
where duplication of the genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
occurs without mitosis
Mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
(cell division).
The extreme in polyploidy occurs in the fern
Fern
A fern is any one of a group of about 12,000 species of plants belonging to the botanical group known as Pteridophyta. Unlike mosses, they have xylem and phloem . They have stems, leaves, and roots like other vascular plants...
genus Ophioglossum
Ophioglossum
Ophioglossum is a genus of about 25-30 species of Ophioglossales in the family Ophioglossaceae, with a cosmopolitan but primarily tropical and subtropical distribution. The name Ophioglossum comes from the Greek, and means "snake-tongue".Adders-tongues are so-called because the spore-bearing stalk...
, the adder's-tongues, in which polyploidy results in chromosome counts in the hundreds, or, in at least one case, well over one thousand.
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.Mixoploidy
Mixoploidy refers to the presence of two cell lines, one diploid and one polyploid. 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.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
Doubled haploidy
A doubled haploid is a genotype formed when haploid cells undergo chromosome doubling. Artificial production of doubled haploids is important in plant breeding....
, which are homozygous and used for genetic research.