Polyploid means having more than the diploid number of chromosomes. Having one chromosome of each type is HAPLOID, two of each type is DIPLOID, three of each type is TRIPLOID, then TETRAPLOID, etc.

There are many forms of polyploidy which we can classify as:

Aneuploidy: The possession of extra chromosomes but not a full extra set.

Euploidy: The possession of one or more complete extra sets of chromosomes. We can subclassify this type of polyploidy as:

Autopolyploidy: the possession of three or more homologous sets of chromosomes. eg an autotriploid could be described as (AAA).

Autopolyploidy can result from failure of the chromosomes to separate at meiosis (complete non-disjunction).

This will lead to diploid pollen or ovules.

Allopolyploidy: the possession of two or more non-homologous sets of chromosomes. An allotetraploid could be described as (AA) (BB).In this case the chromosomes are from two different species of organism. Many plants have developed from ovules fertilised by pollen of a different but related species. It is rare amongst animals, the mule, zedonk (zebra/donkey), tigon (tiger/lion) are examples, but they are infertile. In plants where chromosome doubling often occurs spontaneously the allopolyploid can become fertile.

Alternatively, failure of chromatids to separate during mitosis can lead to an area of tetraploid tissue which if it gives rise to sexual organs will give rise to diploid pollen and ovules. These events occur naturally but can be induced by use of drugs such as colchicine which inhibits spindle formation. The fusion of a diploid gamete with a haploid gamete will give rise to a triploid offspring which will generally be sterile (eg commercial banana plants). Should a pair of diploid gametes fuse then a tetraploid offspring is produced and this will be fertile, although the fertility is often reduced. Generally polyploid plants grow with an increased vigour.

Allopolyploidy is generally more important in terms of commercial application. The fusion of diploid gametes from the two different species results in an amphidiploid (a double diploid) and this will generally be fertile since each chromosome has a homologue. Artificial crossings are the result of colchicine treatment. Many of our crops are allopolyploid, eg wheat, oats, cotton, tobacco, brassica crops, groundnut, strawberry, coffee. Bread wheat is a hexaploid type (7 chromosomes x 6 = 42 in total), strawberry is octoploid (7 chromosomes x 8 = 56 in total).

A new crop known as triticale has been developed by crossing a tetraploid wheat with diploid rye to give a sterile triploid which was then doubled artificially to give a hexaploid. The plant has the nutritional value of wheat and the hardiness of rye. Recent evidence for the organised arrangement of chromosomes within the nucleus which appears to govern their degree of expression and therefore the degree to which they resemble one or other parent should be mentioned when discussing allopolyploidy. The chromosome number in angiosperms ranges from 4 to 264, much greater than in animals and probably resulting from the greater tendency for plants to increase their chromosome number by polyploidy.