ince the double helix discovery 50 years ago, people have been haunted by fears of what scientists might do with their growing genetic knowledge. Some fears have been shared by the scientists themselves. Some have been a bit far-fetched.

Among the scariest prospects have been recombinant DNA research ("gene splicing"), cross-species genetic engineering (mixing human DNA into animals), genetically modified crops (known derisively as "Frankenfoods"), genetic enhancement (inserting supposedly desirable genes into embryos) and germ line gene therapy (changing the genes in the egg or sperm).

The most recent concerns involve embryonic stem cell research and human cloning, which some members of Congress want to regulate or ban.

When the scientific world first grappled with the potential problems of genetic technology, the approach was very different.

"Our experience with recombinant DNA research is the best model," said Dr. LeRoy Walters, a professor at the Kennedy Institute of Ethics at Georgetown. "It showed us that when you have an area of uncertainty, with possible hazard to the public health, it's possible to regulate the research by having it all come before an advisory group."

He was referring to a highly unusual attempt at scientific self-regulation. In 1974, a group of scientists involved in gene splicing, then a new enterprise, became concerned about the possible dangers of their work. They wanted to step back and see how dangerous their research really was.

A committee of 11 prominent biologists sent a letter to the journal Science saying that some of their work raised a "concern for the possible unfortunate consequences of indiscriminate applications." They suggested stopping the experiments "until attempts have been made to evaluate the hazards." They wanted, in effect, a moratorium on the DNA research that worried them.

"I've had people tell me that what we did was the highest form of ethics," said Dr. Paul Berg, a professor emeritus at Stanford and the chairman of the group that proposed the moratorium. "But when you honestly don't know the implications of your research, what are you supposed to do? Just keep on doing it? No, you have to stop and ask yourself what the hazards really are."

The most hazardous research was suspended voluntarily after Science published the letter. Seven months later, 150 molecular biologists met at the Asilomar Conference Center in Pacific Grove, Calif., to figure out what to do next. How risky were these experiments? How could the risk be controlled, without shutting down the century's most promising research?

The meeting, now known as the Asilomar conference, ended with recommendations for containment strategies. The National Institutes of Health created the Recombinant Advisory Committee, which had to review every grant involving potentially risky recombinant DNA.

Most of the fears proved groundless, said Dr. Berg, a Nobel laureate in chemistry. As a result, he said, the restrictions gradually eased, and within a few years, most projects did not require prior approval by the advisory committee, which now oversees gene therapy.

After Asilomar, scientists never again policed themselves in quite the same way. External controls dominate the field today. Some restrictions come with federal grants. Some come from advisory groups, which study thorny issues — usually ethical rather than scientific — and occasionally see their advice turned into federal regulations, especially on research financed with tax dollars.

Scientists often welcome this kind of federal guidance. But when the government refuses to finance certain research, there may be no such regulations. In those regulatory vacuums, some scientists manage to get private funds and do as they please.

Congress has also had a hand in regulating genetic research. The legislative route is now being taken with one of the most incendiary issues of the DNA era: human cloning. Four bills are now before Congress. One, introduced this month by Senator Orrin G. Hatch, Republican of Utah, with five co-sponsors, would ban cloning for reproduction but not for research.

The others would ban all forms of cloning, no matter what the intent; one of these passed the House Judiciary Committee on Feb. 13. It is expected to come before the full House this week, although its prospects in the Senate are not good. Under the bill, the creation of human embryos for stem cell research is defined as cloning and would be banned.

The President's Council on Bioethics has proposed a ban on experiments intended to create baby clones (so-called reproductive cloning) and a four-year moratorium on cloning experiments involving the lab cultures that are meant to treat degenerative diseases (sometimes called therapeutic or research cloning).

But if the ban goes into effect, said Dr. Walters of the Kennedy Institute, scientists could face criminal penalties for engaging in therapeutic cloning. "A federal ban at this time would be premature," he said.

Reproductive cloning, however, is the topic that has captured the public's imagination — and been the source of the public's horror. Still, it is a term widely misunderstood.

While a clone will have the same DNA as the person being cloned, DNA isn't everything. Other than the genome, all else about the clone and the donor — the egg, the uterus, the parents, the social environment — will be different.