In this method, after the egg is fertilized outside the womb, the embryo is allowed to reach the eight-cell stage of development before a cell is removed. This cell is then tested for genetic components that would predispose the child to a particular disease such as Huntington's. Then, only those embryos that do not contain the disease gene are transferred to the uterus, thereby eliminating the chance of having a child with Huntington's disease. Whatever the promise of this technique, the cost is far from trivial. Even in the rare cases when IVF expenses are paid by health insurance, the genetic component is not covered.
Costs are also likely to be high in the even more advanced procedures now being proposed.
In the future, PGD might identify candidates for emerging techniques such as constructive genetic surgery and embryonic cell cloning. Constructive genetic surgery involves removing the affected gene from an embryo and replacing it with normal genetic material. But this is risky business with a failure rate of 80 percent-far too perilous to perform on a single human embryo. Through cell cloning, however, scientists could make multiple copies of the embryo they wish to modify, increasing the genetic surgical success rate. Indeed, cellular cloning seems to hold the key to the successful genetic engineering of humans.