What We Know—And Don’t Know—About Human Heredity

22:11 minutes

Many family photos against a white wall
Family photos from Kaleucagys, Kekova, Turkey. Photo by Anton Watman, via Shutterstock.

Have you ever taken a peek at your family tree? If you trace back along those branches, you might discover some long ago celebrities, kings, and philosophers among your ancestors.

But what does it even mean to be “related” to an ancient queen when it’s hard to know what’s lurking inside our own DNA? It turns out even one generation back, the question of who we are gets complicated. “We’re primed to think of our genomes as some kind of magical book. We just understand so little about genetics. Period.” says Carl Zimmer, author of the new book “She Has Her Mother’s Laugh: The Powers, Perversions, and Potential of Heredity.” Zimmer joins Ira to discuss Mendel’s Law, the history of eugenics, the power of CRISPR and the boundaries of what we understand of human heredity today.

Read an excerpt of “She Has Her Mother’s Laugh” here.

Segment Guests

Carl Zimmer

Carl Zimmer is the author of Life’s Edge: The Search of What it Means to Be Alive. He’s also a science columnist for the New York Times. He’s based in New York, New York.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. Have you ever taken a peek at your family tree? How many branches does it have? Trace your ancestral line back along those infinitely dividing branches, and you might discover that you are related to Emperor Charlemagne. And if you go even further back, you might run into Cleopatra, a direct ancestor perhaps of all living humans.

I know it’s hard to wrap your mind around that. But the math of genetics says it’s true. But what does it mean to be related to the Egyptian queen and what does the term meiosis mean? Where does the idea of race come from, and what does CRISPR do again exactly, and why is it under attack this week?

Well, my next guest says it’s OK that we still have these questions. The field of genetics, the study of inheritance is growing and developing so quickly, that, you know, it is hard to keep up. Well luckily he’s compiled what we know so far into his latest book. So if you have questions about genetics or heredity, I’m certain the answer must be contained in these pages, if there is an answer, that is, and there are a lot of pages in this book.

If you want to talk to Carl Zimmer, science columnist for the New York Times and author of the new book, She Has Her Mother’s Laugh, give us a call 844-724-8255, 844-SCI-TALK. Nice to see you again, Carl.

CARL ZIMMER: It’s great to be here.

IRA FLATOW: The synthesis for She Has Her Mother’s Laugh, why did you call it that, and what was the idea behind it?

CARL ZIMMER: Well, I have been thinking a lot recently about how much we use heredity to define who we are, you know, and we’re always thinking about it, and we’re looking back at our ancestors or you know those of us who have kids are looking at our kids and wondering what do they have of us in them? And, you know, people will say things like, well, she has her mother’s laugh. You know, and what does that really mean?

Well, it’s something that we seize on, and yet I think it’s worth diving deeper and understanding what really is heredity, and why is it so important to us, and what can science tell us about what heredity may actually be.

IRA FLATOW: So what is so what is the difference between heredity and genetics there?

CARL ZIMMER: Well, genetics was a science that was founded to answer the question of heredity. I mean, people have been wondering about ready for a very long time, and they’ve been able to see the effects of it ready for a very long time. And they had various vague ideas about why that was so.

But in the early 1900s, a group of scientists rediscovered this work by some obscure monk named Gregor Mendel, and said, aha, I think we’re on to something here. And they found a science called genetics. And what they said was the science of heredity has now been revolutionized. They thought they had it all figured out.

IRA FLATOW: And then they created tools to change the whole thing, to experiment for the genetics.

CARL ZIMMER: That’s right. That’s right.

IRA FLATOW: I remember, I mean one of the first things I did as a science reporter back in the ’70s was cover the Asilomar Conference, right, which to which we knew how to create these genetic engineering tools. And then people were worried, hey, what can we create that might crawl out of a laboratory? We should stop and talk about this.

CARL ZIMMER: Right, and there, you are talking about basically taking some DNA from one species and sticking it in another. Pretty crude. Now we’re talking about things about CRISPR where basically you can rewrite the DNA of that species and maybe be able to write whatever you want.

IRA FLATOW: And we talk a lot about CRISPR on this program, and I know we were thinking about talking about the CRISPR issue this week that came out, and I thought, I’d save it for Carl Zimmer. He’ll be able to explain it to us. There’s some problem that has developed in CRISPR. Tell us about that.

CARL ZIMMER: Well, the basic idea with CRISPR is that you design molecules that can zero in on a particular spot in DNA, cut it. And then if you want, you can replace what you cut with some new DNA, and this might be a way to treat disease. It’s already proving to be a way to create new varieties of crops because these changes are then inherited. This is a way of changing heredity.

But there have been some questions about whether this is going to be safe and reliable and precise. And just this week, there was a study that suggested that it could be possible to cut out the DNA that you wanted to cut, but maybe cut a lot of other DNA right around it, and if you cut out an essential gene while you’re trying to rewrite something else, you’ve got problems.

IRA FLATOW: Because there are people who are waiting standing by to have CRISPR as an experiment.

CARL ZIMMER: There are clinical trials that could be ready to go now. They’re spring loaded, you know. I mean, for like sickle cell anemia. Like this could really revolutionize medicine, but we’ve got to make sure that it’s safe.

IRA FLATOW: Let’s talk about– delve into your book a little bit more. There’s so much history in the story of genetics, and you take us through a number of the characters. Who would you say is someone who has influenced the field that maybe we haven’t heard who is influential in this? You mentioned Mendel. We’ve all heard of Mendel.

CARL ZIMMER: Right, well you know, there are lots of people who have been long been forgotten. So for example, there was there was a sheep breeder in the 1700s named Bakewell, who actually created an entirely new breed of sheep, the new Leicester people were amazed, like, how did he do that? How is that possible? And what he had been doing was he had actually been very carefully breeding the sheep and bringing in sheep from other farms, and he would take them into these barns where no one could see what he was doing, and it was almost like magic.

So one noblewoman refer to him as the man who invented sheep, and he inspired so many people. Darwin was completely obsessed with how he had created a new breed of sheep. And he influenced Gregor Mendel. And yet, you know, the story of people like him get forgotten, because we have a very– we have a kind of simplistic view of genetics and heredity and how it was all figured out, but it’s actually the history is really fascinating.

IRA FLATOW: And that was a really interesting part that you mentioned, forgotten in history, because over the years as someone who has read a lot about heredity and about genetics, I remember reading way back, I’m sure you read the same stuff, about eugenics starting with the Nazis, and I’m reading in your book, and you’re saying no, it didn’t originate with them. It goes further back, which I hadn’t heard about.

CARL ZIMMER: Yeah. No, I mean if you want to look for a place where eugenics really got off the ground, I mean, one place that I write about a lot in the book is Vineland, New Jersey, a town in Southern New Jersey, where there was a school for the quote unquote feeble-minded. And there was a psychologist there, who introduced intelligence testing in the early 1900s, and became convinced that feeble mindedness was completely hereditary and created these great big family trees to prove it.

And that led to legislation for sterilizing people in the United States, and then the Nazis took a look at this kind of eugenics in the United States and borrowed a lot of their rhetoric.

IRA FLATOW: Let me get out of the way, because I am getting questions about this, because I did mention that we could probably all trace our history back to Cleopatra. How can we all have her as a common ancestor?

CARL ZIMMER: Well, it’s just the geometrical properties of our family trees. You know, like you think about your family tree, and it’s you, and then it branches to your parents and then their grandparents. Now, if you just kept branching it like that forever, it wouldn’t take very long before you had more ancestors than humans who have ever lived.

So actually, your parents are not totally unrelated. They’re cousins. So really, if you were to trace their ancestry back far enough, you’d get to a common ancestor of them. And that’s true for all of us. And you can actually look at the shape of the family tree, just based on mathematical analysis, and you can prove that if you go back a few thousand years, if there’s somebody who was alive then, who has a living descendant today, they must be the ancestor of all living people today.


CARL ZIMMER: You generate that through a mathematical proof. You don’t have to go look at everyone’s DNA. That just comes out of the nature of our genealogy.

IRA FLATOW: Speaking of genealogy, I know you got your genome sequenced while writing this. But you got your whole genome. It wasn’t like sending a little spit in a tube to somebody, right?

CARL ZIMMER: Right, right. It was like 1,000 times more than 23 & Me basically.

IRA FLATOW: And so were you surprised about what you learned?

CARL ZIMMER: I was relieved that I didn’t discover anything horrific in there. But once I was past that, then I was like OK, well what now. So I got hold of the raw data and took it to various scientists and said show me how you study genomes by looking at mine, and tell me what you can find in there.

For example, I got a complete list of my Neanderthal genes, which was fascinating.


CARL ZIMMER: Yeah. Well you know, people who have ancestry outside of Africa are all around 1% Neanderthal, but we inherit different fragments of Neanderthal DNA from interbreeding that happened maybe 60, 70,000 years ago. So you have Neanderthal DNA, I have it, but we probably don’t have the same.

Now mine, I’ve discovered some of my genes have been associated with a risk of nosebleeds. I don’t know why Neanderthals would get nosebleeds, but you know, in people who have it today, there is a slightly elevated risk of nosebleeds.

IRA FLATOW: 844-724-8255, 844-724-8255 is our number. You can also tweet us @scifri. I remember because I’ve been around for a lot of the talks as genetic engineering and genes and heredity have been going on over the years, the early ideas about genes, or at least, in the modern day was that there’s going to be a gene that tells us about everything, one gene, one trait.

There’s going to be an intelligence gene. There’s going to be a gene for sex, for gender. That has not panned out, has it?

CARL ZIMMER: No, absolutely not. I mean that was kind of low hanging fruit. You know, your blood type, one gene, sure. But beyond that, you know, something like height, for example. Height seems simple, right? It’s just, you know, you just take a tape measure to measure it.

It turns out that height has been linked now to about 3,000 different genes, 3,000 different sites in our DNA that are variants that influence how tall we get. And scientists are going to find thousands of more they expect.

So even something as simple as height is influenced by all sorts of parts of our genome. And so there are genetic influences on things like our personality and intelligence, but again, that influence is going to be so complicated. It’s going to take a long time to sort out.

IRA FLATOW: The tweets are coming in from what you’re talking about here. Ray says, “No way, a lost tribe from the Amazon at directly related to Cleopatra. Ask him.”

CARL ZIMMER: Look, what can I tell you? I mean, you can think about well, all these people have been isolated, but really, it only just takes one person to make contact with a group of people to connect family trees together, and then you know, over future generations, those family trees get more and more connected. I know it sounds very counter-intuitive, but you know, just– it would be impossible for it to be otherwise.

And you mentioned Charlemagne before. I mean, Charlemagne lived more recently, but if you look actually in Europe, you can be pretty confident that all living Europeans are probably a direct ancestor, descendant of Charlemagne. So you know, what I think is interesting about all that is that it kind of shows that we shouldn’t like try to make ourselves feel very special by finding a famous ancestor, because we have a lot of company.

IRA FLATOW: 844-724-8255. You know, we are constantly balancing our nature versus nurture discussion, right? I’m sure you’re familiar with it. But does epigenetics, the factors in the environment that influence the expression of genes, cloud that debate, or does it crystallize that debate?

And you know, Lamarck, you know where I’m going with this. You know, we all talk about Darwin, but Lamarck had his counter views that the environment somehow influenced what happened. Could he may be be making a little bit of comeback with epigenetics here? Somehow the environment influencing how genes are expressed and that’s carried through?

CARL ZIMMER: So I think epigenetics is one of the most fascinating aspects of heredity now, and I have a whole chapter where I talk about it, and what makes it so fascinating is that the prospect that maybe experience can be passed down. I mean, maybe you can– maybe that can influence future generations. And you know, there is really solid evidence in some species, in plants, and certain species of animals.

When it comes to mammals and especially humans, I would say that the evidence is much, much murkier. Part of that is because we still don’t really understand how epigenetics really works. It is really very much the frontier of biology.

But it’s exciting, because it does raise these possibilities that you know, what happens in your life might affect your grandchildren.

IRA FLATOW: Talking with Carl Zimmer, science columnist for the New York Times and author of the new book, She Has Her Mother’s Laugh. Don’t be lulled into complacency by that title, because this book is chock full of all kinds of interesting stuff, and it’s 600 pages. Any question you’ve ever wanted– how did you limit to 600 pages?

CARL ZIMMER: There are so many great stories. I mean, I just I would– and I would discover new people, and I’d just be so fascinated by them, people who figure prominently in the history of heredity or people who are doing science right now. It was just so much fun to write this.

IRA FLATOW: So here’s a tweet coming in says, “Do you suppose there are more subtle hereditary things, like the preference for art or music or the seaside or food or stuff like that?”

CARL ZIMMER: Well, I think that you could–

IRA FLATOW: If you could have your mother’s laugh, why can’t you have those?

CARL ZIMMER: Well, you know, it’s going to be hard for you to dissect that. Do you have a thing for art because your parents took you to museums all the time, or because there was some art gene? Like how would you know the difference?

I mean, that’s the real challenge with trying to figure out what it means for you as an individual. But what scientists do is they study entire populations and try to see like, well, how do we account for the differences? So are our relatives more similar than you’d expect compared to other people, and you know what the fact is, with things having to do–

Well, you know, divorce, you can actually argue is what scientists call heritable. You know, that likelihood of divorce is more similar if you’re related than not. That doesn’t mean there’s a divorce gene. That means that there might be genes that influence your behavior in very subtle ways that we don’t understand at all and then maybe in certain situations, tip the balance a little towards taking a certain decisions.

But we’re so far from really having a grasp of that, that it’s just pure speculation.

IRA FLATOW: Because people, as they say, going back, you know, to the early days when we wanted to find one gene, people were looking for that single gene for either homosexuality or sports or something like that.

CARL ZIMMER: Yeah and you have to remember that genes aren’t like people. They don’t do things. They’re just sitting there. And the effects that they have depend on your environment. So there can be genes– you know, I have a gene I discovered getting my genome sequence that actually on average makes people a few pounds heavier than they would be without it. And I have two copies, one from each parent.

And you know, the fact is that for people who were born in the 1940s or earlier, it doesn’t have that effect at all. And it probably just has to do with the fact that people of my generation are growing up in this crazy diet environment, where we can eat all the sugar and carbohydrates we want. And that just wasn’t true before.

And now that gene is becoming a risk for weight gain and obesity.

IRA FLATOW: Well, we’re going to talk more with Carl Zimmer after the break. He’s a science columnist for the Times and author of the new book, She Has Her Mother’s Laugh. If you want to get in on the conversation, our number’s 844-724-8255. Also, you can tweet us @scifri. We’ll be right back after this break. Stay with us.

This is Science Friday. I’m Ira Flatow. We’re talking with Carl Zimmer, author of the new book She Has Her Mother’s Laugh about the science of heredity, and you can give us a call 844-724-8255. Let me see. We’ve got some calls and some tweets. Let me go to the tweets. I know this is going to be a hard one for you to answer. How would you go about– how do I go about getting my entire genome sequence, Donald wants to know. You’re a reporter. You could do it a little easier than most people, right?

CARL ZIMMER: So I did it through a program that– actually I don’t think, it no longer exists. I think they discontinued it just a few weeks ago. But there are some companies. There’s one called Veritas. There’s another company called Helix you might want to look into. Now these places will sequence your genome, but I’m not clear on whether they will actually let you have the raw data.

In other words, they’ll look at it and tell you all about it, but if you want to play around with it yourself, that can be a challenge, because genome sequencing is kind of in a gray zone right now. And there’s a lot of questions about, well, about liability, because genome sequencing is not a precise science. You know, it’s rough and there’s a lot of error correction that has to go into giving you an analysis.

And so if somebody just gives you the raw data, and you misread it and think that you have inherited a gene for some incredible disease that you don’t have or you think you’re safe from some disease, and you go sue them, there could be trouble.

IRA FLATOW: A couple months back, the Golden State killer brought up questions about this privacy issue for people using these family tree databases. Do you think these new tools are– are we opening up a Pandora’s box? Especially youth, you get your total genome sequence, could it leak out? We know now you’re susceptible to nosebleeds.


IRA FLATOW: I mean, could that bite you a little bit later from an insurance firm or something?

CARL ZIMMER: I have put my whole genome online.




IRA FLATOW: Scary, I would think.

CARL ZIMMER: I’m personally OK with it. And, you know, a scientist who I worked with, said like, hey, this would actually be a cool teaching tool. So we’re actually using my genome, an analysis of it in a class at Yale, where every year, these students take a look at my genome and try to find something interesting. But you know, I had the choice there, and so I think it’s important that people have control over their genetic information.

And you know, if somebody uploads their own information about their DNA to one of these genealogy sites, you know, in a way, you’re actually sharing some of your siblings’ DNA or your cousins, because you share a lot of identical stretches of DNA. And we saw that with the Golden State killer. The only reason that a person was arrested is because some of his DNA matched some of his cousin’s.

IRA FLATOW: Let’s go to the phones. Let’s go to Stephen, in Dalton, Ohio. Hi Stephen. Welcome to Science Friday.

STEPHEN: It’s in Dayton, Ohio.

IRA FLATOW: Oh I’m sorry.

STEPHEN: No problem. My question is–

IRA FLATOW: Like the Wright brothers.

STEPHEN: They’re just flying overhead right now. The question I have is about the mutability of the genome in regard to environmental stressors about turning on or turning off genes.

IRA FLATOW: How does the environment turn on or off the gene? How does epigenetics work?

CARL ZIMMER: Well, basically, you know, you have the same set of genes basically in all your cells. But obviously, you know, a skin cell is a lot different than a cell in your brain, and they do different things. And the reason is because they are keeping certain cells shut down and certain ones turned on. And they started doing that when you were an embryo, and that has continued ever since.

But on the other hand, we do have genes that we use to respond to things in the environment. And we have switches that can turn them on, and then in some cases, keep them on. And so there can be experiences early in life that may possibly be able to trigger long term changes that stay that way later in life in ourselves. And so this might be an explanation for how experiences early in life can have effects decades later in terms of your health and behavior and so on.

So we need to be able to respond to the environment, and epigenetics seems to be a really important part of that connection.

IRA FLATOW: And we’re thinking maybe some of those epigenetic changes can be passed down perhaps from generation to generation?

CARL ZIMMER: So that’s the big question is like could somehow epigenetic changes in your own body get passed down to the next generation? Obviously, they would have to get into sperm and eggs, and then they would have to survive into that embryo, and then the next generation. And I don’t think that the evidence is really there for our species. For some other species, yes, which is fascinating.

IRA FLATOW: This is a fascinating book. We’ve just scratched the surface with Carl Zimmer, author of She Has Her Mother’s Laugh, about the science of heredity. It’s a great piece of work. I can see a lot of work going into that, Carl. Thank you for taking time to be with us today.

CARL ZIMMER: Thanks so much.

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