Question: Why can some species cross breed but their offspring are not fertile?! (eg. a horse and a zebra can haev a foal but it can't reproduce) if it can work once, why not twice?

Christina Pagel answered on 20 Jun 2013:

apparently these hybrids have a mixture of their parent animals chromosones (the things that hold all our genes). When two animals from the same species mate, their chromosones pair up together so that the offspring have genes from both parents. They think that for hybrid animals, the chromosones (becasue they’re a mixture) can’t pair up with chromosones from other hybrids (because they’re a slightly different mixture)…