An example is Down's syndrome which results from trisomy of chromosome 21. A chromosome number that is more than two complete chromosome sets. Triploids may be produced by fertilization of an abnormal diploid egg produced by nondisjunction of all chromosomes. Tetraploidy may result if a diploid zygote undergoes mitosis without cytokinesis. Subsequent normal mitosis would produce a 4N embryo.
Polyploidy is common in plants and important in plant evolution.
Polyploids occur rarely among animals, and they are more normal in appearance than aneuploids. Mosaic polyploids, with only patches of polyploid cells, are more common than complete polyploid animals. Chromosomes which lose a fragment lacking a centromere will have a deficiency or deletion . Crossing-over error is another source of deletions and duplications. Crossovers are normally reciprocal, but sometimes one sister chromatid gives up more genes than it receives in an unequal crossover.