I think Orks were a blend of Elf, Maia, Man, Beast and Maia, all blended with the exceptional skill of Melkor- we must remember that as a semi-deity, Melkor possessed vast powers and an intelligence that we meagre humans cannot fathom, to him genetic manipulation would be trivial and he could thus mix the above races to his heart's content. The Story of Andreth hints as those powers and Morgoth's Ring p. 409ff gives Tolkien's later thoughts on the matter.
NT