Editing Ploidy (section)

===Haploid and monoploid===
[[File:Difference of Haploid and Diploid Gene Regulation in Mendelian Genetics.svg|thumb|A comparison of [[sexual reproduction]] in predominantly haploid organisms and predominantly diploid organisms.<br /><br />'''1)''' A haploid organism is on the left and a diploid organism is on the right.<br />'''2 and 3)''' Haploid egg and sperm carrying the dominant purple gene and the recessive blue gene, respectively. These gametes are produced by simple mitosis of cells in the germ line.<br />'''4 and 5)''' Haploid sperm and egg carrying the recessive blue gene and the dominant purple gene, respectively. These gametes are produced by meiosis, which halves the number of chromosomes in the diploid germ cells.<br />'''6)''' The short-lived diploid state of haploid organisms, a zygote generated by the union of two haploid gametes during sex.<br />'''7)''' The diploid zygote which has just been fertilized by the union of haploid egg and sperm during sex.<br />'''8)''' Cells of the diploid structure quickly undergo meiosis to produce spores containing the meiotically halved number of chromosomes, restoring haploidy. These spores express either the mother's dominant gene or the father's recessive gene and proceed by mitotic division to build a new entirely haploid organism.<br />'''9)''' The diploid zygote proceeds by mitotic division to build a new entirely diploid organism. These cells possess both the purple and blue genes, but only the purple gene is expressed since it is dominant over the recessive blue gene.]]

The term '''haploid''' is used with two distinct but related definitions. In the most generic sense, haploid refers to having the number of sets of chromosomes normally found in a [[gamete]].<ref>{{cite web |title=MGI Glossary |url=http://www.informatics.jax.org/glossary/haploid |website=Mouse Genome Informatics |publisher=The Jackson Laboratory |access-date=6 July 2019 |location=Bar Harbor, Maine}}</ref> 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.{{citation needed|date=May 2023}}

An alternative usage defines "haploid" as having a single copy of each chromosome – that is, one and only one set of chromosomes.<ref>{{cite web |title=Talking Glossary of Genetic Terms |url=https://www.genome.gov/genetics-glossary/haploid |website=National Human Genome Research Institute |access-date=6 July 2019}}</ref> In this case, the nucleus of a [[eukaryote|eukaryotic]] cell is said to be haploid only if it has a single set of [[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.{{citation needed|date=May 2023}}

[[Gamete]]s ([[sperm]] and [[egg cell|ova]]) are haploid cells. The haploid gametes produced by most organisms combine to form a [[zygote]] with ''n'' pairs of chromosomes, i.e. 2''n'' chromosomes in total. The chromosomes in each pair, one of which comes from the sperm and one from the egg, are said to be [[homologous chromosomes|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.<ref>{{Cite web |date=23 September 2021 |title=Homologous chromosomes |url=https://www.genomicseducation.hee.nhs.uk/glossary/homologous-chromosomes/ |access-date=10 March 2023 |website=Genomics Education Programme}}</ref>

All [[plant]]s and many [[fungus|fungi]] and [[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]]. Most fungi and algae are haploid during the principal stage of their life cycle, as are some primitive plants like [[moss]]es. More recently evolved plants, like the [[gymnosperm]]s and [[angiosperm]]s, 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]].{{cn|date=December 2023}}

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''',<ref>Langlet, 1927.</ref> also known as '''basic''' or '''cardinal number''',<ref>Winge, 1917.</ref> or '''fundamental number'''.<ref>Manton, 1932.</ref><ref>{{cite journal | author = Fabbri F | year = 1963 | title = Primo supplemento alle tavole cromosomiche delle Pteridophyta di Alberto Chiarugi | url = https://books.google.com/books?id=2ZTzAAAAMAAJ&q=%22fundamental+number%22 | journal = Caryologia | volume = 16 | pages = 237–335 }}</ref> As an example, the chromosomes of [[common wheat]] are believed to be derived from three different ancestral species, each of which had 7 chromosomes in its haploid gametes. The monoploid number is thus 7 and the haploid number is 3&nbsp;×&nbsp;7&nbsp;= 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''&nbsp;= 6''x''&nbsp;= 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''&nbsp;=&nbsp;3''x'').<ref>{{cite web|url=http://mcb.berkeley.edu/courses/mcb142/lecture%20topics/Amacher/LECTURE_10_CHROM_F08.pdf|title=LECTURE 10: CHANGES IN CHROMOSOME NUMBER|website=Mcb.berkeley.edu|access-date=2022-03-10}}</ref>

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''&nbsp;=&nbsp;''n''&nbsp;=&nbsp;23.