Hybrid sterility can be regarded as yet another form of interspecific sterility. The offspring of the interspecific crosses are mainly sterile. Geological studies have shown that the chromosomes of the hybrid individuals fail to synapse at the time of meiosis and thus result in either non-production of gametes or defective gametes. Unless the chromosomes of parents are accurately separated which might result in viable gametes,, in most cases the gametes are not produced and if produced they may not be fertile. The commonly cited example is the mule, a sterile animal the product of a cross between a donkey and a horse. Further, the hybrid species in general are found to have grossly abnormal reproductive system. If normal reproductive system is present, then meiosis is abnormal and non-viable gametes are produced. Thus various types of reproductive isolating mechanisms are at work in different groups of organisms to maintain the distinctness and uniqueness of species.