Summary What is Euploidy? Euploidy refers to the variation in the complete set of chromosomes in a cell or organism. Euploidy is common in plants and occurs in high frequency than animals. Since chromosomal number in a cell affects the sex balance of animals, euploidy in animal cells results in sterility. Hence, euploidy is often related to plants more than animals. During euploidy, the entire set of chromosomes is duplicated once or several times during cell division.
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Etymology[ edit ] The term ploidy is a back-formation from haploidy and diploidy. Diploid therefore means "duplex-shaped" compare "humanoid", "human-shaped". Polish botanist Eduard Strasburger coined the terms haploid and diploid in These gametes are produced by simple mitosis of cells in the germ line.
These gametes are produced by meiosis, which halves the number of chromosomes in the diploid germ cells. 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.
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. 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. 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. 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.
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. 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. Diploid[ edit ] 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 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. 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. 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.
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. It is possible for polyploid organisms to revert to lower ploidy by haploidisation. In bacteria and archaea[ edit ] Polyploidy is a characteristic of the bacterium Deinococcus radiodurans  and of the archaeon Halobacterium salinarum.
Variable or indefinite ploidy[ edit ] 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. Though polyploidy in humans is not viable, mixoploidy has been found in live adults and children. It is a major topic of cytology. Dihaploidy and polyhaploidy[ edit ] Not to be confused with haplodiploidy where diploid and haploid individuals are different sexes.
Dihaploid and polyhaploid cells are formed by haploidisation of polyploids, i. 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. 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 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 are missing an X chromosome.
Aneuploid karyotypes are given names with the suffix -somy rather than -ploidy, used for euploid karyotypes , such as trisomy and monosomy. Homoploid[ edit ] Homoploid means "at the same ploidy level", i. 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. The zygoid state of a species may be diploid or polyploid. 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[ edit ] More than one nucleus per cell[ edit ] In the strictest sense, ploidy refers to the number of sets of chromosomes in a single nucleus 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 ,  though usually the ploidy of each nucleus is described individually.
For example, a fungal dikaryon with two separate haploid nuclei is distinguished from a diploid cell in which the chromosomes share a nucleus and can be shuffled together. This is an important evolutionary mechanism in both plants and animals and is known as a primary driver of speciation. 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 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 gametes are haploid for their own species, but triploid, with three sets of chromosomes, by comparison to a probable evolutionary ancestor, einkorn wheat. For example, species of Xenopus African toads form a ploidy series, featuring diploid X.
Haplodiploidy[ edit ] Ploidy can also vary between individuals of the same species or at different stages of the life cycle. In humans, only the gametes are haploid, but in many of the social insects , including ants , bees , and termites , 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 species, haploid individuals of this species have a single chromosome and diploid individuals have two chromosomes.
Tissue-specific polyploidy[ edit ] 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 as an aspect of cellular differentiation. 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.
Glossary of ploidy numbers[ edit ] Term.
Euploidy: Meaning and Types | Cell Biology
Etymology[ edit ] The term ploidy is a back-formation from haploidy and diploidy. Diploid therefore means "duplex-shaped" compare "humanoid", "human-shaped". Polish botanist Eduard Strasburger coined the terms haploid and diploid in These gametes are produced by simple mitosis of cells in the germ line. These gametes are produced by meiosis, which halves the number of chromosomes in the diploid germ cells. 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.
Euploids are further of different types — monoploids, diploids and polyploids. In monoploids there is a single set of genome, in diploids there are two sets of genome and in polyploids there are more than two sets of genome Table Types of Euploidy: Monoploidy and Haploidy: Monoploid individuals have single basic set of chromosome, e. In flowering plants, the diplophase or the sporophytic phase dominates; the haplophase or gametophytic phase is normally limited to the pollen grains and the embryo sacs.