The African clawed frog (Xenopus laevis) is allotetraploid, likely as a result of an interspecies mating long ago, followed by a duplication of the entire genome. Xenopus laevis is fertile and has a normal life cycle. In contrast, mules, the allodiploid offspring of a male donkey and a female horse, are generally sterile. Why can Xenopus reproduce and mules cannot?

Xenopus laevis is allotetraploid: it has 2 sets of homologous chromosomes originally from one species and 2 sets of homologous chromosomes originally from another species, making a total of 4 sets of chromosomes. During meiosis, every chromosome has another homologous one with which it can pair, producing viable gametes that have the complete genetic information of the species.

Mules, however, are allodiploid: they have one set of chromosomes from a donkey and another set from a horse, making a total of 2 sets of chromosmes. But those two sets are not homologous, so during meiosis the chromosomes can't pair and the gametes will have errors that make them inviable, making mules sterile