Designer babies

Suppose parents-to-be could guarantee their children will grow up to be unusually healthy. Or extra smart. Or maybe just a little better looking than mom and dad.

Sound pretty good?

Now, suppose that guarantee requires a level of planning that goes way beyond the usual prenatal care. Suppose it requires some fiddling with the future kids' DNA, adding a few genes here and there to slow down aging or rev up the brain circuitry or lock in resistance to viruses.

Still sound good?

Even if your answer is a definite no, some scientists believe many parents will find this a very attractive option. For now, the choice is science fiction -- but just barely so.

The time is coming, many scientists say, when parents will pick their children's genes.

From the menu of possibilities, parents might select genes to make their babies resist common diseases and infections, things like cancer, AIDS, heart attacks and Alzheimer's disease. Maybe they would like their children to have fabulous memories or winning personalities or a talent for playing the piano.

A couple of extra inches of height and a thick head of hair could be nice, too.

To hear these scientists talk, all of this and much more will be possible in the not-so-distant future. ''There's nothing beyond tinkering,'' says Lee Silver, a Princeton biologist.

Not this year or next, probably, and maybe not in the next 10 or even 20 years. But these scientists predict the amazing breakthroughs in genetically engineering lab mice and farm critters will eventually be applied to the animals at the top of the food chain.

''It's not a question of 'if' but 'when' and 'how' this will occur,'' says Gregory Stock, head of the Program on Medicine, Technology and Society at UCLA's School of Medicine.

Stock and Silver are visionaries in their field, men who enjoy painting the big picture of an over-the-horizon science called human germline engineering. Germline refers to the sperm and egg. These scientists are talking about changing the genetic makeup of a person-to-be at the moment of conception.

Something like this: Insert a block of new genes into a freshly fertilized egg. The one cell becomes two, then four, then eight. Each new version carries the extra information. In nine months, a baby is born. Every cell in his or her body contains the extra genes.

The child grows up. Marries. Passes the extra genes on to the next generation of babies. And they on to theirs. And so on. Or maybe not. The unsettling prospect of handing these genetic fixes down the generations is just one of the many controversies of this obviously hot-button field.

''The reason people are fascinated by this whole area is that it will challenge our fundamental thinking about who we are and what it means to be human,'' says Stock. ''We are talking about remaking human biology.''

But what part of biology to remake first? Typically the answer is to reduce our tendency to get sick.

While personal habits and medical care play an obvious role in health, inheriting good genes gives some folks a powerful edge. Scientists already know some of the combinations of genes that help people resist some big-ticket illnesses. So one goal of human germline engineering could be to help the genetically less fortunate share these built-in health advantages.

For instance, the risk of heart disease depends in part on the levels of HDL, the good cholesterol. More is clearly better. In the human body, a gene called ap0-A1 makes a major piece of HDL. The same is true in mice, whose biology, scientists love to point out, is not so different from ours.

''It's possible in mice to dial in virtually any HDL level you want by introducing more copies of this gene,'' says Dr. R. Sanders Williams, a cardiologist at the University of Texas Southwestern Medical Center.

So why not add some extra apO-A1 genes to one-cell persons-to-be and reduce their chance of dying from humanity's leading killer?

Perhaps resistance to the AIDS virus would also appeal to gene-shopping parents. Scientists can imagine a way to do that. Those who are born with two defective copies of a gene called CCR5 can escape HIV infection despite thousands or risky sexual encounters. The reason: CCR5 makes a protein that the AIDS virus locks onto when it invades white blood cells. So, no CCR5 protein on the surface of a cell -- no infection.

Of course, there is no reason to stop with disease protection, the visionaries say. Many genetic ''enhancements,'' as they are called, can also be imagined.

One obvious enhancement is extra brain power. At Princeton, scientists have already created mice -- nicknamed ''Doogie'' after TV's physician prodigy -- that are rodent geniuses. They learn faster, remember longer and adapt to changes better than any ordinary mouse.

What makes these mice unique is an extra copy of a gene that produces a brain chemical called NR2B. This stuff boosts the cellular switches that help the brain store associations. (When you remember the name that goes with a face, that's an association.) While a human brain is more complex than a mouse's, the basic machinery of learning may be pretty much the same in both.

Even selecting a child's personality in advance might be possible. Experts believe that half of people's personality traits are hard-wired by their genes. Of course, lots of genes combine to create any individual's melange of quirks and temperament. So building a child with, say, David Letterman wit, Mother Theresa compassion and Warren Buffett business sense may not be real easy at first.

Still, scientists are laying the foundation. In mice, at least, they have already tracked down genes that influence many habits, including aggressiveness, overeating and mothering instincts.

As fascinating as mouse personalities may be to scientists, soon there should be much more information about the genetic underpinnings of such things in larger, less furry creatures. The federal Human Genome Project is deciphering the 100,000 or so genes spelled out by the 3 billion letters of the human DNA library.

Actually, there is no single genetic blueprint, because genes come in many variations. Otherwise everyone would look exactly alike.