Ploidy
When a human germ cell undergoes meiosis, the diploid 46 chromosome complement is split in half to form haploid gametes.
For example, a person with Turner syndrome may be missing one sex chromosome (X or Y), resulting in a (45,X) karyotype instead of the usual (46,XX) or (46,XY).
[b] Some authors suggest that Strasburger based the terms on August Weismann's conception of the id (or germ plasm),[14][15][16] hence haplo-id and diplo-id.
The two terms were brought into the English language from German through William Henry Lang's 1908 translation of a 1906 textbook by Strasburger and colleagues.
In the most generic sense, haploid refers to having the number of sets of chromosomes normally found in a gamete.
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.
[19] In this case, the nucleus of a eukaryotic cell is said to be haploid only if it has a single set of chromosomes, each one not being part of a pair.
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.
[28] However, some genetic studies have rejected any polyploidism in mammals as unlikely, and suggest that amplification and dispersion of repetitive sequences best explain the large genome size of these two rodents.
In plants, this probably most often occurs from the pairing of meiotically unreduced gametes, and not by diploid–diploid hybridization followed by chromosome doubling.
[42] These two species are highly resistant to ionizing radiation and desiccation, conditions that induce DNA double-strand breaks.
[citation needed] The term "dihaploid" was coined by Bender[48] to combine in one word the number of genome copies (diploid) and their origin (haploid).
A human cell with one extra set of the 23 normal chromosomes (functionally triploid) would be considered euploid.
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.
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.
The gametes are haploid for their own species, but triploid, with three sets of chromosomes, by comparison to a probable evolutionary ancestor, einkorn wheat.
[citation needed] Tetraploidy (four sets of chromosomes, 2n = 4x) is common in many plant species, and also occurs in amphibians, reptiles, and insects.
[55] Over evolutionary time scales in which chromosomal polymorphisms accumulate, these changes become less apparent by karyotype – for example, humans are generally regarded as diploid, but the 2R hypothesis has confirmed two rounds of whole genome duplication in early vertebrate ancestors.
However, in many situations somatic cells double their copy number by means of endoreduplication as an aspect of cellular differentiation.
[60] There is continued study and debate regarding the fitness advantages or disadvantages conferred by different ploidy levels.
[63] However, polyploidization is associated with an increase in transposable element content[64][65] and relaxed purifying selection on recessive deleterious alleles.
If the fertilization of human gametes results in three sets of chromosomes, the condition is called triploid syndrome.
Mable 2001 finds Saccharomyces cerevisiae to be somewhat inconsistent with this hypothesis however, as haploid growth is faster than diploid under high nutrient conditions.
This result is also more complex: On the one hand, under phosphorus and other nutrient limitation, lower ploidy is selected as expected.
Thus the NLH – and more generally, the idea that haploidy is selected by harsher conditions – is cast into doubt by these results.
[70] In further support of the masking theory, evidence of strong purifying selection in haploid tissue-specific genes has been reported for the plant Scots Pine.
[69] The common potato (Solanum tuberosum) is an example of a tetraploid organism, carrying four sets of chromosomes.
[citation needed] However, commercial potato crops (as well as many other crop plants) are commonly propagated vegetatively (by asexual reproduction through mitosis),[71] 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.
Some eukaryotic genome-scale or genome size databases and other sources which may list the ploidy levels of many organisms:
1) A haploid organism is on the left and a diploid organism is on the right.
2 and 3) Haploid egg and sperm carrying the dominant purple gene and the recessive blue gene, respectively. These gametes are produced by simple mitosis of cells in the germ line.
4 and 5) Haploid sperm and egg carrying the recessive blue gene and the dominant purple gene, respectively. These gametes are produced by meiosis, which halves the number of chromosomes in the diploid germ cells.
6) The short-lived diploid state of haploid organisms, a zygote generated by the union of two haploid gametes during sex.
7) The diploid zygote which has just been fertilized by the union of haploid egg and sperm during sex.
8) Cells of the diploid structure quickly undergo meiosis to produce spores containing the meiotically halved number of chromosomes, restoring haploidy. These spores express either the mother's dominant gene or the father's recessive gene and proceed by mitotic division to build a new entirely haploid organism.
9) The diploid zygote proceeds by mitotic division to build a new entirely diploid organism. These cells possess both the purple and blue genes, but only the purple gene is expressed since it is dominant over the recessive blue gene.
