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True polyploidy rarely occurs in humans, although polyploid cells occur in highly differentiated tissue, such as liver parenchyma, heart muscle, placenta and in bone marrow. [ 63 ] [ 64 ] Aneuploidy is more common.
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. [citation needed] It is possible for polyploid organisms to revert to lower ploidy by haploidisation. [citation needed]
Aneuploidy is the presence of an abnormal number of chromosomes in a cell, for example a human somatic cell having 45 or 47 chromosomes instead of the usual 46. [ 1 ] [ 2 ] It does not include a difference of one or more complete sets of chromosomes .
The results from polyploid and diploid cells should be identified and separately recorded from one another. This is because the fitness cost (survival to next generation) of chromosomal instability is lower in polyploid cells, as the cell has a greater number of chromosomes to make up for the chromosomal instability it experiences. [5]
Polyploidy is also a well known source of speciation, as offspring, which have different numbers of chromosomes compared to parent species, are often unable to interbreed with non-polyploid organisms. Whole genome duplications are thought to be less detrimental than aneuploidy as the relative dosage of individual genes should be the same.
[5] [6] Aneuploidy can be full, involving a whole chromosome missing or added, or partial, where only part of a chromosome is missing or added. [5] Aneuploidy can occur with sex chromosomes or autosomes. [citation needed] Rather than having monosomy, or only one copy, the majority of aneuploid people have trisomy, or three copies of one chromosome.
Polyploidy in animals is much less common, but it has been significant in some groups. [43] Polyploid series in related species which consist entirely of multiples of a single basic number are known as euploid. Haplo-diploidy, where one sex is diploid, and the other haploid. It is a common arrangement in the Hymenoptera, and in some other groups.
The list of organisms by chromosome count describes ploidy or numbers of chromosomes in the cells of various plants, animals, protists, and other living organisms.This number, along with the visual appearance of the chromosome, is known as the karyotype, [1] [2] [3] and can be found by looking at the chromosomes through a microscope.