Techniques of genetic modification allow scientists to change an organism's genetic code -- creating a plant or animal that never before existed. Not surprisingly, this idea frightens many people. They fear innumerable unforeseen possibilities, including creatures or diseases against which we have no defense. Many scientists strongly acknowledge these fears, saying that despite all we know about genetics, there is still much that remains shrouded in mystery. So what is possible, and what will likely never be?

By introducing genes into newly fertilized eggs, geneticists can give plants and animals traits that would never be possible within their species, genus, or even scientific kingdom. Indeed, scientists have placed insect genes in the cells of mammals, and grown plants that carry animal genes. Just a few examples are potatoes that deliver a virus-fighting vaccine to those who eat them, beets that produce a low-calorie sweetener in place of sugar, mice that make human growth hormone in their bladders, and goats that produce as a by-product of their milk a protein that's helpful to hemophiliacs.

The technique of producing these so-called "transgenics" owes to the fact that all organisms -- animals, plants, bacteria -- have a huge number of genes in common. And we now know where many of these genes are located and what they do. This means that scientists can find, for instance, a portion of an animal's genome that is instrumental in the formation of a certain body part, and insert genes that will change that body part in sometimes-dramatic ways. This is not to say, however, that the process is simple, or that the possibilities are limitless.

For now, and likely forever, the idea of swapping whole body parts across species exists only in the realm of science fiction. There is no master gene for wings, for example, and so no simple genetic swap that would cause a frog to sprout dragonfly wings instead of large legs for jumping. Complete structures like limbs and hearts and eyes are extremely complex, relying on countless genes and proteins to create their structures and drive their functions. For the time being, introducing just one gene that codes for a single protein is difficult enough.