Our understanding of our genetic makeup is being greatly expanded by a systemmatic mapping process known as the Human Genome Project, carried out internationally with enormous commercial and government funding. Smaller projects are also drawing the genetic map of pigs, chickens and some other organisms. As this work proceeds, individual genes are being identified for various functions and especially for medical conditions. Sometimes it appears that a single gene is responsible, for example in cystic fibrosis, but most conditions seem to be caused by more complex sets of factors, both genetic and environmental. We should make an important distinction between a gene which causes a condition outright, and one that gives one a susceptibilty to it, but which requires other factors to be present as well for the condition to develop. The ability to detect such genes now means we can use the tests for screening, especially pre-natally. This raises some important ethical questions, as we shall see.
Screeing for various diseases is not, strictly speaking, the same thing as manipulating or "engineering" them. Some people feel "engineering" is an inappropriate term, with connotations of cold mechanics rather than living things, but it does reflect that manipulating genes has in some cases become a relatively common laboratory technique. We should also put it in context, that genetic engineering has been performed for centuries in animals and plants by selective breeding. This enhances particular genetic traits based on outward appearance, by choosing, for example, which boars to mate which sows to develop, over many generations, leaner pig meat.