Aneuploidy : One or inore chron~osomes of a normal set nlay bc lacking or present in excess. The term riullison~y refers to absence of both the chromosomes of a pair. Monosomy, trisomy, tetrasomy etc., refer to the occurrence of a given chromosome once, thrice, four times ctc., resycctively in a diploid organism.
Polyploidy : Refers to the occurrence of more than two sets of chromosomes in an individual. Most organisms are diploid, that is they have two sets of chromosomes in their somatic cells, but only one in their gametic cells. Polyploid organisms would be triploid when they have three sets of chromosomes, tetraploid if they have four sets and so on. Polyploidy occurs frequently in plants but is a rare phenomenon in animals. Polyploidy has been a common means of speciation in plants. Many of thc cultivated plants, such as wheat, oats, tobacco, potato, banana, coffee, sugarcane and many flowering plants are polyploids.
Plant breeders always attempt to induce polyploidy by artificial means to create new varieties. Polyploidy can be regarded as thc only means in bisexual organisms by which a new species could arise in one step.