Researchers usually make a distinction between polyploids that arise within a species and those that arise due to the hybridization of two distinct species. The former are known as autopolyploids, while the latter are referred to as allopolyploids. Autopolyploids are essentially homozygous at every locus in the genome. However, allopolyploids may have varying degrees of heterozygosity depending on the divergence of the parental genomes. Heterozygosity is apparent in the gametes that polyploids produce. Allopolyploids can generally be distinguished from autopolyploids because they produce a more diverse set of gametes (Figure 2).
Different species exhibit different levels of tolerance for polyploidy. For example, polyploids form at relatively high frequency in flowering plants (1 per 100,000 individuals), suggesting that plants have a remarkably high tolerance for polyploidy. This is also the case for some species of fish and frogs. However, higher vertebrates do not appear to tolerate polyploidy very well; in fact, it is believed that 10% of spontaneous abortions in humans are due to the formation of polyploid zygotes.