When National Geographic caught up with Zimmer at Yale University, he explained why it’s impossible to predict what skin or hair color Prince Harry and Meghan Markle's children will have, how a book by an American geneticist underpinned Hitler’s mass extermination programs, and why your Neanderthal DNA may have positive health benefits.

When I told my wife the title of your book, and that it was about heredity, she said: “I got my mother’s laugh and voice from being around her.” So, Carl, is it genes or the environment—nature or nurture—that make us who we are?

No one has done a rigorous genetic study of the genetic inheritance of laughter. [laughs] But researchers have studied behavior in general, and genetics, which show that there are different kinds of behavior and aspects of people’s personalities that are “heritable.” This means that if you look at the variation in a trait in a big group of people, some of that variation is due to the genes that they inherit. Identical twins, for instance, will tend to be more similar to each other in that trait than their siblings.

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But I would be surprised if laughter had even 10 percent heritability. Genes you inherit may play a role in your laugh being somewhat similar to your parents’, but you’re also growing up with them and listening to them laughing, and we’re a very imitative species. There’s no way you could drill down and say we have identified that 10 percent of your laugh came from your DNA. We’d like it to happen. That’s why consumer genetic tests are incredibly popular. Somehow we want to look inside our DNA and get a precise measurement of why we are the way we are.

Certainly our genes are enormously important, but they’re not the only things that are passed down from our parents. I would argue that you should think beyond genes when trying to understand the full scope of heredity. For example, we humans are cultural animals and culture works like its own form of heredity. You’re probably already discovering, for example, that your children or grandchildren are a lot better than you are at using a smart phone. They may go on to think about a new way of designing phones, which will then get passed down to future generations.

There has been much comment on the marriage of Prince Harry, who is of German-Greek ancestry, and Meghan Markle, whose mother’s forbears were African slaves. Is there any way we can predict what their children are going to look like?

We tend to fixate on differences between people, so there’s a lot of fascination with the marriage of Harry and Meghan or her having African ancestry. But if you look at their full complement of DNA, it’s practically identical. There are huge stretches where there’s nothing but a sprinkling of difference between them. But there’s a sprinkling of genetic difference between Harry and anybody else in Great Britain. The human species is fairly genetically uniform.

How people look depends on a lot of different genes. Eye color, hair color, skin color—they all have their own sets of genes that influence them. Each person carries two copies of each of those genes, and in some cases you need two copies of the same version, to give you red hair, for example.

In 2017, it was discovered that Neanderthals and humans interbred during their coexistence. Take us back in time and explain how Neanderthal genes may be helping us fight disease.

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Neanderthals probably existed for several hundred thousand years in Europe and Asia; their last fossil record is from about 40,000 years ago. Our own species evolved in Africa and the oldest Homo sapiens fossils are about 300,000 years old. Since the beginning of our species, we probably expanded into Neanderthal range and interbred. In some cases modern human DNA ended up in Neanderthals and Neanderthal DNA ended up in our own gene pool.

The most Neanderthal DNA people have is just a couple percent. But since there are billions of people with Neanderthal DNA, there’s a lot of Neanderthal DNA on Earth today. [laughs] You can pretty much rebuild the Neanderthal genome from all the fragments floating around inside us today.

Initially, Neanderthal genes were probably harmful because they weren’t well adapted to cooperate with our own genes. This may have had effects like harming our health or making it hard for us to have children. It does seem that some Neanderthal genes stuck around, though, and spread because they are beneficial. One possibility is that there may be genes that help our immune systems. Maybe Neanderthals evolved to resist certain kinds of pathogens and people who inherited those tended to survive more than people who didn’t.

It may be that when you go for a health check in the future, the doctor might say, “Unfortunately you have this Neanderthal variance, which makes us concerned that you’re going to have a higher risk for high cholesterol.” Scientists are still trying to find out what the exact health effects are of all these different variants. Most of it is a mystery.

Many readers will be shocked and dismayed to discover that Hitler’s racial program had its roots in America. Tell us about the Vineland Training School and the haunting case of Deborah Kallikak.

In southern New Jersey there was a school built in the late 1800s called the Vineland Training School—originally called the New Jersey Home for the Education and Care of Feebleminded Children. In 1906, a psychologist named Henry Goddard was hired. He wanted to find a scientific way to measure the intellect of students there. He was also discovering genetics. People had just rediscovered Gregor Mendel’s research, and many American geneticists, in particular, felt that this was the explanation for why some people were intelligent and some were not or why some people committed crimes and others did not.

The thinking emerged in the U.S. that you needed to keep people with these bad genes from having children. Henry Goddard used the children at the school as “evidence” for this policy. He investigated their genealogy and claimed he could see these genes being passed down from generation to generation, and that this was proof positive that eugenics was the right thing to do.

Goddard published a book called The Kallikak Family, about a student at the school named Emma Wolverton, whom he called Deborah Kallikak. The book claimed to show how criminality and feeblemindedness travelled through the generations. The implication was that you have to keep these people from reproducing or it would be terrible for the U.S. He also advocated sterilizing women to keep them from reproducing if they were deemed unfit. As the Nazis grew more powerful they grabbed onto American eugenics, and the Kallikak family was used as proof that unfit people should be kept from having children. They then went beyond sterilization and began exterminating people based on this kind of justification.

Goddard’s thesis turned out to be completely false. He claimed that a distant ancestor of Deborah Kallikak, a soldier in the Revolutionary War, had a fling with a feebleminded girl, who had a son and then produced a line of feebleminded people, including Deborah. This soldier then went on to marry an upstanding woman and for generations their children were all productive members of society. He would show these family trees, where one side was all black circles and squares showing they were feebleminded or criminal, and the other side was all white, meaning that they were noble and admirable. Only decades later did a couple of researchers go back and look at public records and piece together the real history of this family. And it bore no resemblance to what Goddard was showing. His whole genealogy was a total fraud.

My son is six foot four and towers over me. But the science of how height is inherited is surprisingly complex, isn’t it?

Height seems like it should be a simple thing that we would have figured out a long time ago. It’s just a number you get from a ruler. [laughs] Yet, scientists have been puzzling over height for over 150 years and they’re still a long way from deciphering it. We do know that height is very strongly heritable, which means that in a population, a lot of the difference is going to come down to genes. But it doesn’t mean that just one gene influences how tall you are. Scientists have already identified over 3,000 genes in DNA that influence height, and that will probably go up to 10,000 and beyond.

That’s partly because height depends on all sorts of different things, like how we take in energy or how we convert that energy into new cells. So, once you dive into something as simple as height, you start to realize, “Oh, gee!” This heredity stuff is fascinating in its complexity. The fact that your son is tall could have something to do with the genes he inherited, but it could also have to do with the time and place where he was born and grew up. The whole world has gotten taller! [laughs]

In recent years, something known as epigenetic heredity has hit the headlines. Explain what it is and how the popular press has tried to use it to explain crime in America.

We inherit genes but how those work depends not just on the sequence of our DNA, but on the molecules that surround our genes and switch them on and off. The study of them is called epigenetics. There’s some fascinating evidence that, at least in some species, the epigenetics gets passed down from generation to generation, much like genes do. For instance, if a plant has experienced drought, it could change its epigenetics to make it more drought tolerant, which it could then pass down to its descendants.

Unfortunately, in popular culture epigenetics has run way ahead of the evidence. Lots of people claim that you can change your own fate by taking control of your epigenetics. You can even take epigenetic yoga if you like. [laughs] Some people have claimed that traumatic experiences can change people’s epigenetics, which then gets passed down to children and is supposed to explain problems in terms of crime and so on. That’s dangerous because you’re starting to use pat biological explanations for complicated social problems.

A geneticist declared in 2015: “We are close to being able to alter human heredity.” Tell us about CRISPR and the ethical dilemmas that may arise from our intervention in one of nature’s most complex mechanisms.

CRISPR is a scientific tool that has burst on the scene in the past few years and has proven very useful for editing DNA. Some scientists have even used it to alter the DNA inside early human embryos. The possibility emerges that we could program in genetic changes to future children, and those changes could then be inherited by their descendants. You might want to do that to relieve people of serious risks of disease, but there are all sorts of other possibilities that emerge.

Do you want to reduce their chances of getting very common diseases, or enhance them in some way, so those future generations don’t get a choice in the matter, while you do? It raises all sorts of ethical issues about equity, for example. Is it fair for some people who can afford it to alter their future generations? These are issues that we are only just starting to grapple with. But they’re going to come at us really fast!

This interview was edited for length and clarity.