Why interspecific crosses are rare in nature and intergeneric crosses almost unknown ?

Explanation

In interspecific crosses, male and female animals of two different related species are mated. The resultant progeny may combine desirable features of both the parents are infertile. Thus are rare in nature. The same applies to intergenric corsses. It is the crossing of two different animals/plants of different genus. It is almost unknown in nature as the gametes show species specificity.