The ancestor of modern commercial wheat is believed to be an interspecies hybrid of two species of Triticum, T. monococcum and T. searsi, which contributed chromosome sets A and B, respectively. In animals, such hybrids are typically sterile (cf. Horse x Donkey sterile male Mule or female Jenny). In plants, however, they may lead directly to a stable  autopolyploid by genome duplication, here an AABB tetraploid that produces AB gametes. A cross between this tetraploid an another diploid species, T. tauschii (chromosome set D) produced a triploid (ABD), which again underwent a genome duplication, and produced modern T. aestivum, a hexaploid with an  AABBDD chromosome set. The hexaploid chromosome set has become diploidized, which is to say that homologous chromosomes in each of the A, B, and D sets diverge genetically from each other so that they behave separately during meiosis.