Ploidy refers to the number of complete sets of chromosomes in a cell. You’ll probably have heard of haploid (n, gametic cells) and diploid (2n, somatic cells) cells in the human body so this is just delving a little deeper.
The number of chromosomes in a cell (it’s genome) = x
Diploid cells (somatic cells) = 2x
Haploid cells (gametes of a diploid organism) = x
We also know:
Gametic chromosome number = n
Somatic chromosome number = 2n
This is different from above as n may contain a number of complete genomes (therefore several x’s).
A polyploid cell is one which contains a number of genomes:
Diploid = 2x. Therefore 2n (somatic cells) contain 2x genomes and n (gametes) contain x genomes.
carrying this on…
Tetraploid = 4x. 2n = 4x and n = 2x.
Hexaploid = 6x. 2n = 6x and n = 3x. (eg common wheat is an allopolyploid with 6 sets of chromosomes)
Octaploid = 8x. 2n = 8x and n = 4x.
If each genome in a cell is identical, it is autopolyploid.
But if the genomes in a cell differ (are not identical, come from different ancestry) it is allopolyploid.
Variation in the number of chromosomes per genome:
If the cell has the same number of chromosomes per genome present (therefore a balanced set of chromosomes) then it is Euploid.
However, if we have more or less of a single chromosome (eg diploid cell has 3 copies of one chromosome rather than 2) it is Aneuploid. This would be caused by non-disjunction of an individual chromosome and is often lethal in animals (although there is natural aneuploidy in insect sex chromosomes where female = xx and male = xo).
With aneuploidy there are a number of well known genetic diseases, one of which is Down’s syndrome caused by trisomy 21 (three copies of chromosome 21).