Ploïdie

The ploïdie of a cell characterizes the number of specimen of its chromosomes:

a cell is Haploïde if it has $n$ chromosomes.
It is Diploïde if it has $2n$ chromosomes organized in $n$ even.

More rarely, one can meet:

of the cells or the triploid developmental stages ( 3n chromosomes)
of the species Polyploid S whose chromosomal inheritance is the double of the normal (tétraploïdie = 4n ), triple (hexaploïdie = 6n ), even more
of the anomalies of the number of chromosomes or Aneuploïdie S

General outline

Fecundation and meiosis

At the species with sexuée reproduction (animal, plants, mushrooms,…), the fusion of the two cells or Gamète S provided by each parent at the time of the Fécondation assembles two batches morphologiquement identical of chromosomes, those of the male gamète and those of the gamète female. The egg or Zygote which results from fecundation thus contains two identical batches of chromosomes.

These chromosomes being appariables, one indicates the number of chromosomes of the egg, which is thus said diploïde, by 2n . The gamètes which do not have that only one batch of N chromosomes are known as haploid.

During the development of an organization, the normal cellular divisions or Mitose S preserve the number of chromosomes of the cell: a haploid cell gives two haploid cells, a cell diploïde gives two cells diploïdes.

To ensure the complete life cycle (of egg X with egg x+1), it is thus necessary to compensate at one time or with the another chromosomal doubling of fecundation by an operation reverses, the Méiose, a particular cellular division, which allows a return at the haploid state.

According to the groups of living beings, the place of the meiosis and fecundation are variable during the development cycle. For example, the meiosis takes place during the formation of the gamètes at the Mammifère S with the result that all the cells (resulting from egg by Mitose S) are diploïdes and that only the gamètes are haploid. In other groups like certain mushrooms, the meiosis following fecundation immediately, all the cells are haploid.

Ploïdie and phase of the life cycle

See Life cycle (biology)

Life cycle with diploïdie dominant, known as diplophasic

At the organizations whose life cycle is with dominant phase diploïde, the major part of the cells is diploïde, except for the gamètes.

Majority of the animal (whose Man), are with dominant phase diploïde.

Life cycle with haploidy dominant, known as haplophasic

The organizations whose life cycle is with dominant haploid phase primarily consist of haploid cells, just like the gamètes. The phase diploïde is practically limited to the zygote resulting from fecundation.

Many a Mushrooms, some Algue S are examples of organizations to dominant haploid phase.

Life cycle mixed, known as haplodiplophasic

There exist also organizations with mixed life cycle balanced (certain filamentous green algas have a haploid phase and an equivalent phase diploïde) or unbalanced.

Particular cases

Triploidy

Triploidy is associated with the reproduction of the Angiosperme S. One of the cores of the grain of Pollen (haploid) is linked with a core remained diploïde of the ovule, and gives rise to a fabric of reserve of the Graine, endosperm. It is the phenomenon of double fertilization. There exist also triploid plants like the Bananier or certain varieties of Pommier. These plants are sterile and can be miltipliées only by vegetative way.

Polyploidy

Certain species perpetuated and fixed in their Génome, one or more additional duplications their chromosomes: it is the phenomenon of polyploidy. The load of DNA in the cell is thus strongly increased. That can relate to animals (drosophilas for example) or plants.

The corn is hexaploïde ( Triticum aestivum ) and has a genome of very big size (16000 Mb) which makes 127 times that of the model plant ( Arabidopsis thaliana ). Other species as oysters are tetraploid.

Anomalies of the ploïdie

At the time of fecundation or embryonic development, anomalies of the number of chromosomes can appear, it is the Aneuploïdie. The Trisomie S are aneuploïdies.

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