Ploidy – Variation of Chromosome Numbers

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).