Q. Moral issues aside, since humans and chimpanzees are so genetically similar, can human sperm impregnate a chimp's egg?

R.U.

Arlington

A. The answer from some of the most renowned scientists in the field is a unqualified ``maybe.''

Dr. John Biggers, professor of cell biology at Harvard Medical School, says, ``No one knows because I don't think anyone has tried.'' Such research, he explains, is fraught with moral and even legal risks. And almost no one will fund it.

As to the biochemical question, Biggers says human sperm can probably get through the outer ``shell,'' the zona pelucida, of a chimp's egg. The three-layered zona surrounds the cell membrane of the egg and has receptors on it, like tiny locks. The head of the sperm has proteins that are like specially-shaped keys. The keys and locks of each species are designed so they only fit each other.

Dr. Ryuzo Yanagimachi of the University of Hawaii ran a series of experiments to develop a test of the viability of human sperm by seeing if it would bind with the zona receptors of other species. He didn't try chimps, but he says human sperm did bind with the eggs of gibbons, another primate. The experiment was terminated before the sperm made it into the egg cell.

But he doesn't think the human-chimp combination would work. ``Human sperm is very choosy about its partner,'' Yanagimachi says.

If the sperm-egg binding process is so species-specific, how do you explain hybrids, like mules bred from horses and donkeys? Biggers explains that sometimes when species are similar enough, the proteins in the sperm of one species will fit the zona pelucida receptors in the other. ``They're not absolutely precise,'' he says. ``This may be built into the system to encourage evolution.''

When the key and lock do fit, it triggers the sperm to release enzymes that dissolve the zona in that one spot. The sperm then passes through and enters the egg. That triggers the zona to close all its receptors to any other sperm.

But then the big question arises. Would the chromosomes from the human sperm match those in the nucleus of the chimp egg closely enough so they can fuse, forming new cells and a new organism? The answer, said several renowned geneticists, is, ``We don't know.''

Hybrids have been bred from animals that are further apart genetically than humans and chimpanzees. But until somebody actually dares to try this one, no one will be able to say whether nature can abide a human-zee.