SITTING with his pregnant wife in the obstetrician鈥檚 office, Carl Zimmer stopped the genetics counsellor from giving her lecture on the potential risks of starting a family in your 30s. He wrote about genes for a living, after all, what did he have to learn?
But his smugness soon faded. The more they talked about ancestors of whom he knew little, the more alien he felt his genes become: 鈥淢y mutations seemed to flicker in my DNA like red warning lights. I had willingly become a conduit for heredity, allowing the biological past to make its way into the future.鈥� And yet he had no idea exactly what he was passing on.
Thus starts a voyage to find out where he comes from and what he might deliver down the generations. Zimmer鈥檚 journey is personal: at one point he has his genome sequenced expecting to find something exotic lurking in the shadows, only to be told he is genetically really rather boring.
Advertisement
His book, however, is anything but. It is nuanced, entertaining and balances eloquent storytelling with well-researched science. I should confess that its title, She Has Her Mother鈥檚 Laugh, is pure catnip to a science writer days away from giving birth to a daughter conceived using an anonymously donated egg. But with that caveat, anyone interested in their path through history, and what they may hand on, will find much to excite them.
We tend to think that heredity is why we are like our ancestors, specifically the DNA we gain from our parents and how our genetic past gives rise to the present. Zimmer鈥檚 book raises the possibility that we need to expand such gene-centric notions.
He is not the first to suggest this. He acknowledges researchers such as Marcus Feldman at Stanford University in California, who began work on a theory of heredity in the 1980s that included culture and genes. Then there is Russell Bonduriansky at the University of New South Wales in Australia and Troy Day of Queen鈥檚 University in Canada, who argue in their recent book Extended Heredity that we need mathematical equations to unite genetic and non-genetic heredity in a single description.
鈥淶immer has his genome sequenced expecting the exotic, only to be told he is genetically rather boring鈥�
Zimmer expands on this by exploring key evidence that non-genetic heredity 鈥� for instance, culture, microbes in our bodies and epigenetics 鈥� can work with our genes to affect how and what we inherit from our past.
Take cheese. If you enjoy Brie, you have probably inherited genetic mutations for making lactase, an enzyme that breaks down the lactose sugar in milk. These mutations can be traced back to parts of the world with a deep history of cattle herding. This cultural practice was initially to provide meat, since mammals tend to be lactose intolerant after weaning. But as the milk soured naturally and turned into yogurt and cheese 鈥� foods containing less lactose 鈥� our ancestors may have discovered it was easier to digest.
Natural selection would then have favoured people with mutations allowing them to digest lactose: their ability to use milk as a source of protein and carbohydrates would make them more likely to survive famine. 鈥淚n other words, genetic heredity and cultural heredity ended up pushing in the same direction,鈥� says Zimmer.
The evidence for non-genetic heredity is often thinner, though. Take the fact that consuming alcohol during pregnancy can lead to fetal alcohol syndrome. Emerging evidence indicates that fathers who drink before conception contribute to the disease because alcohol alters which genes switch on and off in their sperm and are then carried into offspring. Research using animals suggests such changes persist over several generations, yet the jury is still out on just how this works.
Zimmer鈥檚 more generous definition of heredity is particularly relevant in light of biotech breakthroughs like CRISPR gene editing, which allows the insertion and deletion of genes to produce permanent changes that can be passed on.
Zimmer elegantly weaves such cutting-edge technology with tales of our ancient ancestors, tracking how our basic knowledge of heredity grew from Aristotle鈥檚 belief that only men could be the true parent of a child, to stories of kings whose incest led to heirs with hearts the size of small nuts, but whose ill health was still seen as the fault of witches.
The stories of our past are pulled closer to our present by way of Emma Wolverton, who in 1897, was delivered to the gate of a school for 鈥渇eeble-minded鈥� children in New Jersey. Her family history soon caught the eye of a scientist called Henry Goddard, who believed that mental incapacity could be inherited and that Emma 鈥� labelled a moron 鈥� provided the perfect proof.
Her ancestors could be traced, said Goddard, to a single man from an upstanding colonial family. Drunk one night, he had slept with a 鈥渟imple鈥� barmaid, before later marrying a woman of 鈥渂etter stock鈥�. The descendants on the barmaid鈥檚 side tended to crime, alcoholism and feeble-mindedness, while the so-called legitimate branch of the family rose to prominence, he said.
Convinced the US was sliding into a crisis of heredity, Goddard wrote: 鈥淚f civilisation is to advance, our best people must replenish the Earth.鈥� Talk of sterilisation, institutionalisation, even a call for 鈥渁 gentle, painless death鈥� to kill 鈥渢he very weak and the very vicious鈥� soon followed.
Emma鈥檚 story ended up forming the basis of a 1912 book that was to fuel modern eugenics, and inspire Hitler鈥檚 crimes. Years later, researchers found that the story of Emma鈥檚 ancestry was wrong. Her family was not directly descended from its supposed founding father. Emma herself was literate, well read and held down several jobs. The story was disproved, yet its legacy lingered.
鈥淐ells from the fetus can also pass to the mother, and even on to subsequent siblings鈥�
Luckily, we have now unpicked many past misunderstandings in favour of complex and novel theories about heredity. For obvious reasons, I was drawn to a chapter on microchimerism, the extraordinary process whereby cells interchange between mother and fetus. Here, Zimmer dispels the myth that heritability works in one direction. A mother鈥檚 cells will often infiltrate any of her future children鈥檚 bodies as they grow in her uterus, where the cells can endure for decades. But cells from the fetus can also pass to the mother, and even on to subsequent siblings.
For instance, one woman was found to have an entire lobe of her liver made up of cells that bore a Y chromosome. These were traced to her boyfriend. She had undergone an abortion years before, and some of the cells from the male fetus remained inside her. When her liver was damaged by hepatitis C, the fetal cells she still carried rebuilt it.
Zimmer treads lightly over more contentious issues, such as why race does not exist and why we are instead a product of 鈥渁 biological blender鈥�. His opinions don鈥檛 contribute much that is new here, but they will help you hold your own in dinner party chat.
Elsewhere, he is more passionate 鈥� in particular, over the ethical minefield surrounding mitochondrial replacement therapy, or so-called 鈥渢hree-parent鈥� babies, a technique to stop deadly mitochondrial disease.
Other modern technology also provides him with food for thought. One day, people who are infertile may be offered the option of turning their skin cells into stem cells capable of creating gametes. And children could be 鈥渄esigned鈥� to possess desirable genetic traits or to avoid hereditary diseases such as cystic fibrosis. Mosquitoes are already being manipulated to pass on genes that prevent their descendants spreading malaria.
Clearly, as Zimmer explains, inheritance is so much more complicated than the handing on of a genetic baton. She Has Her Mother鈥檚 Laugh is, as promised, a showcase of the powers, perversions and potential of what we truly gain from our past and pass on to our future.
Pan Macmillan
This article appeared in print under the headline 鈥淓nlarging the human story鈥�
