Polyploidy describes the case of a cell or an individual possessing entire extra sets of chromosomes. The type of polyploidy is designated by the number of haploid (N) sets that are present. Triploid (3N) individuals have three sets of chromosomes while tetraploid (4N) individuals have four. A karyotype of a triploid individual can be seen here. Note that there are three copies of each chromosome. Polyploidy is extremely common in plants. As a matter of fact, common food plants such as wheat, seedless grapes and bananas are polyploid. However, polyploidy is not common in mammals, and it is usually lethal when it occurs. In humans, polyploidy can be caused by at least two mechanisms: dispermy and unreduced gametes. Dispermy is the term used to describe the process by which two sperm fertilize a single egg to produce a triploid zygote. Unreduced gametes are diploid rather than haploid. The union of an unreduced egg and a haploid sperm would result in a triploid zygote, while the union of an unreduced egg and an unreduced sperm would result in a tetraploid zygote.
Causes of Polyploidy
It may arise naturally or artificially as a result of increase in number of chromosomes sets. If cytoplasm fails to cleave, a tetraploid is formed. A fusion of a diploid and a haploid gametes result into triploid organism while the fusion of two diploid gametes produces a tetraploid individual and also it may occur if an egg is fertilized by more than one male gametes. Autoployploidy induced artificially by Chemicals by colchicine, Radioactive substances and Alternate heat and cold shocks.