When an aneuploid zygote divides by mitosis, it transmits the chromosomal anomaly to all subsequent embryonic cells. Abnormal gene dosage in aneuploids causes characteristic symptoms in survivors. 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.
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. Alterations of chromosome structure Chromosome breakage can alter chromosome structure in four ways. Chromosomes which lose a fragment lacking a centromere will have a deficiency or deletion . Join to a homologous chromosome producing a duplication .