Scientists are preparing to perform tests for defective genes in 3-day-old human embryos, a historic experiment that may raise the question of whether parents should be able to choose the hereditary characteristics of their children.

Using the techniques of in vitro fertilization, researchers at the Howard Jones Fertility Institute at Eastern Virginia Medical School in Norfolk will remove a number of eggs from a woman and fertilize them with sperm in a test tube. When an embryo has grown to four or eight cells, the researchers will remove one of the cells and test its DNA, which contains an identical copy of all the genes in each of the embryo's remaining cells. Only those embryos that appear normal will be implanted in the woman's womb.

The goal of the procedure, which has never been attempted in humans and is planned as early as next spring, is to allow parents to identify serious genetic disorders -- such as Down's syndrome, sickle cell anemia, muscular dystrophy, cystic fibrosis and Tay-Sachs disease -- before an embryo is implanted in a mother's uterus and pregnancy begins. Currently, those diseases cannot be diagnosed until after three months of pregnancy.

Many researchers believe the new techniques eventually will be of much wider significance.

If, as expected, scientists expand the list of diseases and biological traits that can be identified by genetic tests to include, for example, susceptibility to such health threats as high blood pressure and obesity or such physical characteristics as eye color, parents might use pre-implantation tests to plan the biological makeup of their children. They could, for instance, fertilize a dozen eggs, test each resulting embryo for a battery of genetic traits and then begin pregnancy with the one they like best.

This kind of eugenics is not entirely new, of course. Through amniocentesis at the third month of pregnancy, parents have for some time been able to reject, through abortion, fetuses they believe are genetically inadequate. But many researchers believe that giving parents an opportunity to choose among a variety of embryos before pregnancy begins would push the practice of eugenics in a new and, some say, potentially troubling direction.

"This whole business seems to treat human reproduction as something analogous to building a car," said Vanderbilt University's Jean Bethke Elshtain, who has written widely on reproductive technology and is one of many who find the new eugenic possibilities unsettling. "Do we want it to be bright red or bright blue? How do we make it sleeker? It turns reproduction into a technological procedure, more and more divorced from human intimacy and human relationships."

Even those more sanguine about the implications of embryo screening concede that the technology raises ethical questions that demand consideration. Should parental choice be limited to eliminating serious genetic defects? Or does the procedure involve such early embryos -- they are still microscopic specks -- that few people would object to discarding them for such reasons as having an unwanted hair color or being the wrong sex?

"Parents do all sorts of things after birth to enhance their children's characteristics, like vitamin supplements or music camp," said University of Texas law professor John Robertson. "The question that we will have to grapple with is whether what is being done prenatally is morally any different."

The experiment at the Jones Institute, a national leader in in vitro fertilization, is based on the same principle used routinely in amniocentesis. Since all the cells of an embryo at every stage of development contain identical genetic blueprints encoded in their DNA, a cell analyzed at any point during development can, theoretically, provide an accurate forecast of genetic problems.

Removal of a cell from an early embryo should pose no harm, scientists said, because the procedure is known to be safe for other mammals. At that early stage of development, all of an embryo's cells are identical and have not yet been assigned any specialized roles. That is why, for example, an embryo can split at the four- or eight-cell stage and produce perfectly healthy twins.

Two recent technical developments have brought screening before implantation to the verge of reality.

The first is a technology known as PCR (for polymerase chain reaction), which allows scientists to take as little as one cell's worth of DNA and copy it thousands of times, yielding a quantity that can be easily read and decoded. Without PCR, scientists need thousands of cells for genetic tests. For this reason, the current method of screening fetuses must wait until the third month of pregnancy.

The second advance involves a technique to remove one cell from an embryo without damaging any of the others. After testing three techniques on mice and monkeys in experiments whose findings are now being readied for publication, the researchers settled on a procedure known as extrusion: A microscopic, hollow needle punctures the "shell" surrounding the embryo, and one of the cells is sucked out.

The Jones researchers would like to try this on human embryos conceived by couples who risk passing on a serious genetic defect. The experiment would run several years, with researchers monitoring the children repeatedly during pregnancy and after birth to make sure that the genetic screening accurately spotted defective embryos and that the process of testing did not cause harm.

Researchers at the institute said they have been careful in setting up their experiment. Before it proceeds, it must be approved both by an internal review board at Eastern Virginia Medical School and by an outside panel of lawyers, scientists and professional ethicists.

The design of the experiment is also very restricted. Only a few couples will be in the first group. In addition, the screening will look for only one kind of genetic defect -- the one that causes Tay-Sachs disease, a hereditary disorder that leads to mental deterioration and death in young children, most often striking Jews of East European origin. No other genetic information from the embryos will be analyzed.

"I don't think the public has any trouble recognizing that a baby born with Tay-Sachs has anything but a tough row to hoe and the parents and children are all affected," said Gary Hodgen, president of the Jones Institute. "But if we go {too far} in other directions, the public will not trust us." Hodgen said his group has no intention of using the procedure for "trivial" questions such as hair color or sex determination.

But while scientists believe the purpose of embryo selection is to address serious genetic defects, it is not clear that they will be able to limit the technology to that use. Legal experts say, for example, that if the genes for health risks, such as high cholesterol and high blood pressure, or for cosmetic traits, such as hair color, are identified, it will be difficult to prevent prospective parents from demanding tests for those features and using only embryos that pass those tests.

"If one takes procreative choice seriously, then one has to be free to have information that affects it," Robertson said. "Because your decision to procreate or not may depend on the genetic characteristics of an embryo, one's right to procreate gives one a right to that information. That means a state cannot ban amniocentesis. Nor can it ban pre-implantation genetic diagnosis."

However, even if parents are free to do as they choose, Robertson said, some kind of ethical guidelines is needed.

"The position most advisory committees have come to is that we need to treat embryos in a respectful way," he said. "We don't have to transfer {implant} them. But because they are a symbol of future persons, we shouldn't toss them around willy-nilly."

According to some ethicists, that means that parents should permit the creation of multiple embryos for selection only as a way to avoid serious diseases or medical conditions. "Trivial" traits such as hair color, some say, should never be used as the primary criterion for choosing among embryos.

Or consider gender. "It seems to me that if we are serious about treating the genders in a non-discriminate way, it ought to begin as early as possible," said LeRoy Walters, an ethicist at Georgetown University's Kennedy Institute. At the very most, he said, parents should use embryo screening only to balance the sex ratio of their family after they already have had at least one child.

"I think at the level of moral advice, we're going to have to warn each other about the dangers of perfectionism: that even if we can identify certain vulnerabilities and/or tendencies, we have to realize some of those vulnerabilities and tendencies are perfectly compatible with a very happy and with a very good quality of life," Walters added. "I think we also have to recognize that any children that we have, even with the most refined and elegant pre-implantation diagnostic tests, are going to be a mixture of traits that we find to be constructive and desirable and other traits which we wish were not there."