Carl Zimmer Wants Us To Be Generous About Our Definition of Heredity
But don't shell out any money on epigenetic yoga just yet.
In 2003, a woman named Lydia Fairchild got a DNA test as part of her qualification for welfare benefits. She was told that according to her results, she was not the mother of the three children she had grown in her body and given birth to. (Needless to say, she was denied welfare.) Social services also began to ask: Had she kidnapped those children? Was she committing welfare fraud?
At the time, Fairchild was pregnant with a fourth child and after she gave birth—with a court officer as a witness—they drew blood for a DNA test. The newborn baby’s DNA again did not indicate she was its mother.
We often think of heredity in simple terms: you get your DNA, or genes, from your mother and father, who in turn got theirs from their mothers and fathers. As we learn more about the biological mechanism of heredity—and even face the possibility of editing it—there can be a tendency to simplify the concept of inheritance.
It turns out Fairchild was a chimera, meaning her body contained cells with different genomes. In some people, for various reasons, each of their cells does not carry the same genes like our biology textbooks taught us. Sometimes mothers hold onto cells from their sons. In other cases someone might have shared the womb with a fraternal twin and absorbed their cells.
In a new book, She Has Her Mother’s Laugh, by science writer Carl Zimmer, Fairchild's story is just one of many examples he offers to complicate our idea of heredity and its history, and to prove it’s more complex and varied than we might think.
Zimmer delves into heredity’s often ugly past, and shows how the idea of heredity has been used to justify one group’s superiority over another. In the early 20th century, a psychologist named Henry Goddard coined the word “moron” to describe people who came from families that were intellectually inferior. He believed this was a heritable trait, and he helped launch the American eugenics movement. His 1912 book on a family that he felt proved his theories, The Kallikak Family: A Study In The Heredity Of Feeble-Mindedness, was highly influential to the Nazis.
Scientists have since come a long way since then in understanding the nitty gritty of how our DNA works and might influence traits, like intelligence. It took decades of hypotheses and studies to understand the structure of DNA, how cells divide, and how two siblings receive a different combination of DNA from their parents than one another. But Zimmer’s greater point is that we still have a lot to learn.
I spoke with Zimmer about the history of humans trying to understand heredity, examples of heredity that contradict our ideas of family and inheritance, and the question of whether we place too much symbolic value on the passing along of DNA.
You've written about many areas of science, but it seems like genetics has been a favored beat for you. What was your first introduction to genetics and what initially drew you to it?
I think the first time I started really thinking about genetics was when I first learned in grade school about evolution. And it blew my mind that there could be a process that could produce an incredible variety of species on Earth through the changes to DNA. So that over a million years you could go from one form of life to a very different one. And then it wasn't until I became a science writer that I really started to drill into the actual science of genetics and the research the people were doing to figure out how that actually works.
You had your own genome sequenced, and brought it to multiple researchers for interpretation. What’s something you discovered that made a lasting impression?
I was quite blown away when I got a spreadsheet from one of the scientists that had a list of genes on it. There were a few dozen genes, and these were genes in my own DNA that had come from Neanderthals. The fact that we could figure out what genes Neanderthals had was enough. And then on top of that, they're at the point now where they can pinpoint their DNA in living people—people outside of Africa have 1 or 2 percent Neanderthal DNA. That really made me stop for a moment and think about how far we've come in this research.
The irony is that you look at those genes and try to figure out what they're doing to me and my life and it's really hard to figure out any answers to that. And scientists themselves aren't much help. You look at one gene, and it's associated with cell division. Or another gene is involved in the immune system. You say, "Okay. What does that mean?" And they say, "Well, I don't know." And then they can look at the medical literature and say, "Oh. This gene is associated with cell deformity." And it turns out I have a Neanderthal gene that's associated with toe deformity—and when I look at my toes, they look fine. So again, it kind of leaves you in this limbo for now. You have some of this information and yet we're still trying to figure out what it really means. We can trace heredity and yet we still struggle to make sense of it.
Did it make you feel less human to learn you have Neanderthal gene in your genome?
No, I think of Neanderthals as humans. They're human too. And one of the exciting things that I talk about in my book is that there are probably several vanished kinds of humans who left their DNA in us. So there are Neanderthals that we know about from fossils that there are also a mysterious group of people called Denisovans who were probably cousins of Neanderthals who were in Siberia and East Asia. And so there are a lot of ghosts in our past.
Something that struck me when reading about the long history of people trying to understand heredity was how it could serve as a vehicle for prejudice. Heredity could be used to single out why a certain race was inferior, justify the superiority of kings, explain—incorrectly—why the poor had lower intelligence, and encourage laws to prevent certain people from having kids. Why do you think heredity is so malleable to fit people’s biases? Is our current understanding of genetics and heredity fueling any modern prejudices?
I think people have looked for ways to set groups of people apart for a long, long time. And there was a time where they would say, "Well, these people in this country are very passionate and excitable because they grew up in a warm place." And so they would see a difference that was based on their environment. Then you could look at the way people describe others as being under a curse. In the Bible, Noah’s son Ham is cursed, and there’s this idea that Africans inherited that curse as a result. So you have that sort of language of setting people apart deeply embedded in our history.
And then, these ideas start to get explained with scientific concepts. So in the 1600 and 1700's, you start to see scientific racism emerge, where there are these explanations that people are different because they belong to different subspecies called races. Or even different species. And when genetics come in, people grab onto the language of genes to justify a lot of these old ideas all over again. And you still see that today when you have people claiming that somehow research on DNA proves "that races are real." I think they're misunderstanding the science and they're again, trying to keep alive these old concepts.
It’s like trying to claim that the Greeks' four elements are still meaningful. Yeah, there's such a thing as fire and there's such a thing as air, but saying that you would have to divide everything into these four elements, that would be silly. Scientists would really not want to go back to that and they don't want to go back in terms of these divisive ways of looking at humanity.
It seems like we’ve also been long grappling with the heredity of intelligence: how to measure it; if it’s innate or if it can be improved. How does intelligence offer an example of how contradictory the concept of heredity can be? How can we understand how a trait can be both heritable and influenced by environment?
Intelligence is a very powerful and controversial concept. and the more you get to know it the more paradoxical it seems. You can see that there is influence of heredity on the different test scores that people get. Identical twins: they test more similarly than fraternal twins. And at the same time it's far from 100 percent the explanation for all the variation. Also in lots of ways, the environment can intervene and change people's intelligence scores. So you have this contradiction. But in my book, I show how this is actually the rule not the exception when it comes to our nature.
Take height for example. I mean height seems really simple compared to intelligence. It's hard to even define what intelligence is really, but height, you can get out a tape measure and get a number. And it turns out that heredity of height is very similar in describing intelligence in the sense there are a lot of genes that influence how tall people become. Scientists would say that it's heritable. But at the same time, the environment in which you grow up is hugely important to how tall you end up being. All over the world, for the past century, people have shot up several inches. And that's not because the genes changed. It's because we changed our own environments. So I'm hoping by talking about height and intelligence, it helps people think about all of the things that go into being human in a more subtle way.
This seems like a great time to have you define what you call "genetic essentialism."
In the book, I talk about some research that psychologists have done on how we use genes to explain the facts of life. And some researchers have coined this term genetic essentialism, meaning [the belief] that somehow genes are supposed to define our essence, and really when you get down to it, all that really matters is your genes.
One way to show how powerful genetic essentialism is is doing experiments where they have people read about the causes of obesity: Some people will read an article that's talking about environmental influence on obesity—what happens if people exercise and diet and so on and how that can affect obesity rates. The other people just read about research on genetics of obesity, like: "Researchers found that this gene can make you X pounds heavier," and so on, so forth. Just that, nothing else.
And then they would put those people in front of a plate of cookies. It turns out that if you just spend a lot of time reading about how obesity is all in genes, you're going to eat a lot of cookies. You're going to eat more cookies than you would if you read something else.
You also say that we can mistake heredity for kin. In the context of our 23-and-Me era, what do you mean by that? Do we place too much genetic meaning on our far ancestors? For example, tell us who Roger Goodspeed is to you, your genetic relationship, and compare that to your symbolic relationship.
We have a way of equating kinship with sharing genes. That there must be some type of genetic connection we have to our kin. that's why it’s exciting for people to discover that they have a cousin that maybe they didn’t know about. But the problem there is that you haven't shared life with these people. You haven't shared experiences with them, and so all you have is this set of molecules in common. And I think that's kind of problematic. In the book I talk specifically about exploring my own genealogy.
My mother's family is named Goodspeed. Very proper, old name and I can go to an old genealogy book and actually work my way all the way back to the first Goodspeed who came to America in 1630's. Roger Goodspeed. I know where he came from in England, I know where he lived here. I’ve got that very well-documented genealogy. I sort of think of him as a direct ancestor, like I have some sort of connection with him. We're the Goodspeed family. But the fact is that he's ten generations back. If you talk to geneticists about the kind of connection you have with people that far back, they point out to you that actually you either have tiny, tiny little DNA in common with some of our ancestors, or none at all.
It's just the way that DNA gets copied in every generation that only so much gets passed down from generation to generation. Actually after a while, a person's descendants can lose that DNA, lose that genetic connection. So it's about a roughly 50/50 chance that I have inherited no DNA at all from Rodger Goodspeed. So I have no genetic relationship to him. That doesn't take away his part in my genealogy, but trying to depend on genetics to justify it is an illusion.
You give some examples that further challenge the idea of genetics and family. I really liked the examples of human chimeras. Can you tell me what a chimera is?
We tend to think of heredity as just going from one generation straight down to the next. And that's the way genes are passed down through generations. And sometimes it doesn't work that way. There’s an example where a woman is pregnant and cells from the fetus shed off and go into her bloodstream and then go to lots of different parts of her body and can then take hold and develop into tissue there. So there are women who, after they've died, have been autopsied and scientists looked through their brain and they found neurons with Y chromosomes in them. Those are male chromosomes. The only way that could have happened is for their son to basically have passed their DNA from their husbands. These are neurons that are firing away inside of their brains.
People like this are called chimeras, which comes from the ancient Greek term for a monster made from different animals put together. And then there are other chimeras who have had a twin in the womb and that twin, maybe a fraternal twin, didn't survive. No one knew the twin existed and it got absorbed into the other fetus. So the surviving twin has their original cells but also has cells of the twin mixed in there. And these can end up in all parts of the body. They’re a mixture. This is common enough that every now and then, it will get people into a lot of trouble.
If you were a chimera, would it change your relationship with your kids and make you feel different about fatherhood?
I think it would be unsettling to think of yourself as an amalgamation of two individuals. People who are chimeras are totally normal, just like everybody else. And there's a lot more chimeras than we realize. A fair fraction of people are chimeras, it’s becoming clear. But still, when people discover they're chimeras, it can be a difficult thing. To see that there's a difference between your genes and your child's genes can be disorienting. If we get over our genetic essentialism, then I think it's okay and we can recognize that families are more than just a genetic matching exercise.
What is cultural inheritance, and do you think our current fascination and focus with biological mechanisms overshadows this?
We humans are really special because we can do a lot of things that other species can't. One of the things that we're really good at is that we can teach and we can learn from teachers. For other species it really doesn't seem to be the case. There are a lot of anthropologists who have come to believe that that process of teaching things to the next generation, and the next generations being able to build on what they're learned and pass that down—that in itself is another kind of heredity. We can look at it and understand it as heredity in the same way that we look at genes. I mean they have different rules, but things are still being passed down from generation to generation. Cultural heredity, I argue in the book, has been essential to our success in our species.
It was by gradually learning about how to plant crops and how to breed them and so on that the agricultural revolution came about. And how we essentially took over the whole planet through farming, through hunting, through navigation. All these things that would not be easy for you to figure out on your own if you were dropped on an island in the middle of nowhere. We depend on culture. And so we’re here now thanks to that legacy. We do focus an awful lot on genes, and genes are obviously important. But then you’re left with people saying, like, "Well, we're genes plus the environment." That term, “the environment” does get neglected a lot. Well what do you mean exactly? Are you talking about whether you grew up in the city or a farm, or if you're rich or poor? What exactly does that mean?
The fact is that it's a lot harder to understand the environment side than it is to understand genetics. Genetics is not easy but at least you can identify genes, you can use comparisons of those genes between different people. Trying to do a scientific study on environment is a much taller order.
I've always been fascinated with epigenetic inheritance, but you say in the book that the evidence is kind of a mixed bag at the moment. Is that something that you think we should really pay a lot of attention to?
Epigenetics is incredibly exciting and fascinating, and it opens up the possibility of a heredity of experience. We think of ourselves, in terms of heredity, as being sort of containers for our genes. Parents deliver their genes to the next generation and then they pass down further. But people have always wondered, "Well, is it possible for your experience to produce changes that could then be inherited?" And researchers have been studying all the molecules that control our DNA and also passed down when cells divide and it's kind of giving a new lease on this idea that experiences can be inherited.
This field is called epigenetics, broadly. We can definitely see this happening in plants: where what they experience affects future generations, many generations down. There's pretty good evidence in some animals like worms. But when we get closer to ourselves in mammals like rats, it gets more debatable. When we get to humans, a lot of researchers argue that they're isn’t really compelling data yet for it. It'd be amazing if it turned out that children could inherit something that came about through the experience of the parents. Either experiencing stress, or some kind of disease or what have you, and pass that down to their own offspring. But we're just not there yet. And you can see a lot of hype about this sort of thing.
You can even take classes in epigenetic yoga but I think people should save their money and not pay for epigenetic yoga just yet.
After spending all this time challenging our definitions of heredity, how do you think we should expand our notion of what heredity is?
I think we should be generous on our definition of heredity. I think we should think of heredity as what the past has given to the present and what part of the present will be passed down to the future. And that potentially could take a lot of different forms. It could take the form of genes, it could take the form of culture, and maybe other things. Maybe even microbiomes. It's possible that some ideas about a bigger heredity don’t pan out and that's okay. We shouldn't close ourselves off to those possibilities and just have a simplistic view of heredity. That would be a shame.
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