The role played by gonads and their genes in the process of sex differentiation is realized in their absence. Gonadectomy is one of the earliest techniques used to know the role played by the gonads and their hormones in sex differentiation. Because of their rarity and inability to function, the abnormal hermaphrodites, though an important resource, have so far not received much attention. This is perhaps true of the steriles too. Morbid embryos, caused by one or another mutant gene, have long been reported. However, the mutants, especially the viable ones, have begun to receive their due attention by medical and molecular biologists since this millennium. This section broadly indicates their occurrence and importance in our understanding of the process of differentiation including sex differentiation.