When the Mishnah Kilaim dealt with animals, it was mainly concerned about unlike species harnessed together or interbred. Neither the Mishnah nor later Jewish literature prohibited ownership of animals bred in such a manner. Interest in this subject, however, diminished, so there was no Babylonian Talmud to these chapters of the Mishnah and later discussion of this material is sparse.
The chief biblical section that deals with this issue, aside from the legislation mentioned above, is the story in Genesis in which the young Jacob promised to maintain the flock of Laban and as payment asked for the speckled, spotted, and dark-colored sheep and goats. He then proceeded to influence the breeding in that direction. Ostensibly this was done through the placement of shoots of poplar, almond, and plane tree, but there has been some speculation that he possessed some knowledge of genetics which helped him to his goal for a large flock. That theory has been advanced by Judah Fliks(“Yorashah Usvivah Bemaaseh Yaakov Betzon Lavan,” Tehumim, Vo. III pp. 461ff.). We should note that the biblical commentators do not single this story out for special comment and to the best of my knowledge do not use it as an example of animal breeding.
Occasionally commentaries, such as Rambam who stated that human beings should not change nature as that would imply imperfection in God ’s creation.(Rambam to Lev.19:19). That medieval view was found frequently in church literature. It has not been followed by Jewish thinkers.
Jewish law said nothing about changing the characteristics of a particular species or breed. Throughout the centuries every effort was made to assist nature and to produce animals suited to specific purposes as well as plants that would yield abundantly. Despite Jewish involvement in agriculture through the centuries, this matter has not been discussed in the older responsa literature, to the best of my knowledge. In modern times, these efforts have been accelerated through selective breeding and an understanding of the genetic process. More recently, cloning of plant tissues has been used successfully to produce plants that are absolutely true; this method holds great promise as well as potential dangers.
Genetic engineering of plants or animals within a species poses few old halakhic problems, though it raises many other issues. Human beings have selectively bred plants and animals