A hybrid is the offspring of a mating between two genetically dissimilar parents. This often occurs between two related species or varieties. When hybridization is attempted between two unrelated species, the genetic incompatibility is usually too great and the embryo fails to thrive or fertilization does not occur.

Oftentimes, hybrids are sterile, resulting from the failure of the chromosomes to pair up properly during meiosis. The more distantly related the organisms the more likely it is that the offspring will be sterile. Sterile plants are bred using vegetative means of propagation, while sterile animal hybrids must always be bred from the parental types.

Perhaps the most famous hybrid is the cross between a horse and a donkey: the mule. The mule is sterile because the horse has a diploid chromosome number of 62 and the donkey has a diploid chromosome number of 64. Subsequently, the hybrid mule has a diploid number of 63 chromosomes, which are unable to pair correctly during meiosis. The mule is a cross between a female horse and a male donkey. The cross between a male horse and a female donkey is a hinny.