|MadSci Network: General Biology|
Interspecies mating cannot happen in nature, even if it does occur, the progeny is sterile. Fertilized eggs which result from an interspecies mating, most of the time, can not develop into a normal young one. The differences in the genetic make up between parent and the offspring leads to the rejection of the "newly developing embryo" by the maternal immune system as it considers this as a foreign tissue. Even if the embryo develops into an young one (if the fetus has similar cell surface antigens that resemble that of the mother and as a conquence NOT rejected by the maternal immune system), the animal is usually sterile (because of chromosomal anomalies). For example: Hinny is a sterile offspring produced through a mating between a male horse and a female donkey. Mules are produced as a consequence of a mating between a male donkey and a female horse. Note that these mules are also sterile! There are many ways by which different species of animals are isolated from each other and maintained, called as isolating mechanisms.( refer "Evolution: Process and Product" by E.O.Dodson and P.Dodson,ISBN 0-442-22164-9 pages 355 - 375)
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