By Barry Starr

Science fiction is filled with stories about improving people through genetic engineering or selection. And like a lot of science fiction, some of what they talk about is coming to pass right now.

No one is going to be changing an embryo's DNA anytime soon, like the Bene Tleilax of Dune. We just don't have the tools or the know how to pull this off. For the foreseeable future, you're stuck with your DNA.

But what scientists can do right now is screen embryos for desired traits, like in the movie Gattaca. I doubt very much though that there will be some government program designed to perfect the human race though screening. Instead it might become commonplace with parents wanting to help their kids succeed by making sure they had the best genes available. And in most cases, this will be the most foolish choice they can make in raising their child.

Scientists are at a very early stage in understanding what effect different gene versions have on people in different situations. Except for those that lead to genetic diseases like Huntington's or cystic fibrosis, there probably are no "best" or "worst" genes. Many gene versions will be either good or bad depending on how and where a child is raised.

This isn't simply an academic discussion about something we might confront in the distant future. Genetic screening of embryos is something happening right now. As we get better and faster at reading DNA, parents will soon be able to have a complete readout of their child's DNA before that child is implanted in the womb. Parents need to be aware of the limitations of what we actually know about that readout before taking screening too far.

What Scientists Don't Know Can Hurt Your Child

Parents who undergo in vitro fertilization (IVF) have the option of using preimplantation genetic diagnosis (or PGD) to screen embryos for various genetic conditions. After the testing, only those embryos that have the desired genes are implanted. The other embryos are discarded (or frozen away and then discarded).

In many cases this sounds more sinister than it actually is. For example, PGD probably makes sense in situations where both parents are carriers for devastating diseases like sickle cell anemia. The doctor would discard embryos that would develop sickle cell anemia so that the parents wouldn't have children with this deadly disease.

Even in this cut and dried situation there are possible problems. For example, should the doctor discard embryos that are carriers of the disease as well? There isn't any health problem associated with being a carrier and there is the added benefit of being more resistant to malaria (a possible big deal on a warmed globe). But parents may want to protect their children from having to go through genetic screening when they are ready to be parents and so may opt for children who aren't carriers.

The uncertainties become more pronounced as we start to think about more complicated diseases. Imagine that a mom has a history of depression in her family. And lots of people on dad's side of the family have type 2 diabetes.

The parents want what is best for their child so they go through IVF and select embryos that don't have versions of genes that can lead to these two diseases. The children are born and grow into fine people. But we'll never know what could have been.

Like most everything in life, genes aren't simply black and white. Many genes that turn up as bad in one study can turn up useful or beneficial in another. This is because genes are like plants-what they end up becoming depends on the environment where they are grown.

If you plant a rose in the desert, you'll end up with a sickly plant. You might conclude that roses are not very pretty flowers and we shouldn't plant them anymore. So instead you plant something hardy that looks OK. Not spectacular but pretty nonetheless.

But now if you plant the rose where it can thrive, you end up with a beautiful flower. The hardier plant still looks nice but it isn't nearly as stunning.

This same sort of thing can happen with genes. Some people have gene versions where they do fine in most any situation. They are the pretty, hardy flower. But some people have genes that allow that to be spectacular or sickly depending on how they are raised. They are the roses of the world.

One of the best characterized examples of a "rosy" gene is called SERT. This gene comes in two different versions (or alleles)-short (S) and long (L). People with only short versions are at a much higher risk for becoming clinically depressed.

This makes depression sound a bit like sickle cell anemia-two copies of a certain version leads to disease. The parents who have depression on mom's side of the family might decide to screen their embryos and select only those that have at least one long version of the SERT gene. Now their child would be at a much lower risk for depression.

What our parents may have missed, though, is that the increased depression risk happens only when the child is abused. A deeper look at the data shows that people with only the S version who had a happy childhood were actually less likely to be depressed. These folks were more resilient and better able to handle the stress of everyday life.

In other words, people with only the S version are roses. Plant them in the right environment and they'll be spectacular. People with the L version are hardy plants that do fine anywhere.

By striving for genetic perfection, the parents have robbed their children of the chance to be amazing. They've created a competent accountant, not an Einstein.

So this is what happens when our parents try to decrease their child's chances for being depressed by getting rid of the short version of the SERT gene. But what about dad's family history? Remember, his family suffers from type 2 diabetes.

Right now scientists haven't found anything nearly as solid as the SERT gene for type 2 diabetes. They just don't have a good handle on the genetics of this disease yet. But let's do a little thought experiment…

One of the ways that scientists are looking into increasing human lifespan is through something called caloric restriction. They know that if they restrict the number of calories that a mouse or nematode eats every day, these animals can live up to twice as long as normal. The hope is that something similar will be true in people.

Imagine, though, that caloric restriction only works in people with a certain version of a gene. And that in the wrong situation, that gene version can lead to type 2 diabetes.

Since type 2 diabetes studies are easier and faster that lifespan ones, scientists would undoubtedly uncover this gene version's link to type 2 diabetes first. The parents who have diabetes on dad's side of the family don't want their child to develop type 2 diabetes and so they select embryos that lack this gene version.

Much later scientists find out that people with the "diabetes" gene version live longer if they limit the calories they eat. By making sure that their child does not have this gene version, these parents have forced the child to live a shorter life compared to the other kids. But at least the child is at a lower risk for type 2 diabetes!

Just messing with two genes has led to a child who won't do as well in tough situations and who won't live as long. What sort of child would develop if parents selected even more of the child's genes? There are undoubtedly hundreds or thousands of other "rosy" genes out there that can be good or bad depending on the situation.

Rise of the Rosy

Imagine a future where some people can afford genetic screening (and so get it) and most other people can't. Because of rosy genes, screened people lose their chance at being a genius or super athlete.

In this case we might have a future that is the opposite of the one portrayed in the film Gattaca. In that movie, the people who had undergone genetic screening were the ones in power. And through their superior DNA, they were going to stay that way.

Yet because of their selection against rosy genes, we could imagine the screened people might start out in power but be overthrown by the superior unscreened. A rosy future indeed.

Barry Starr, PhD, runs Stanford University's Stanford at the Tech program at the Tech Museum of Innovation in San Jose, California. He is also a Geneticist-in-Residence at the Tech Museum.

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