Chapter 1
Beyond DNA: Unraveling the Tapestry of Inheritance
When Carl Zimmer's wife became pregnant with their first child, a routine visit to a genetics counselor sparked an existential crisis. Confronted with questions about his family medical history, Zimmer realized how little he knew about his own heredity. This moment of panic in a sterile medical office would launch him on a journey to understand the true meaning of inheritance-not just the genes we pass to our children, but the complex web of biological, cultural, and environmental factors that shape who we become.
The resulting book is a masterful exploration that challenges our conventional understanding of heredity. While we often think of inheritance as simply the DNA we receive from our parents, Zimmer reveals a far more intricate reality: we inherit microbiomes, epigenetic markers, cultural knowledge, altered environments, and economic systems-all of which profoundly influence our development and identity. From Habsburg monarchs to modern genetic engineering, from ancient breeding practices to CRISPR technology, Zimmer's investigation spans centuries and disciplines to reveal that the question "What do we inherit?" is far more complex-and fascinating-than most of us ever imagined.
Chapter 2
The Roots of Heredity: From Royal Blood to Scientific Revolution
Long before scientists understood genetics, humans developed elaborate concepts of heredity. Medieval European nobility obsessively documented their genealogies in "Golden Books" and constructed pedigree trees (pe de grue) to visualize their unbroken lineages. Blood became the metaphorical carrier of inheritance, with noble families protecting their "bloodlines" from lower-class "taint." This concept expanded to define "races" sharing blood, with Spanish priest Alfonso Martinez de Toledo claiming a knight's son would naturally prefer swordfighting over farming because "the good man of good race always returns to his origins."
The Habsburg dynasty exemplifies how these beliefs played out in practice. Their determination to maintain "pure" royal blood through marriages between close relatives led to numerous hereditary afflictions, most visibly the Habsburg jaw. Philip II married his first cousin, producing the physically deformed and mentally unstable Don Carlos. As generations passed, Habsburg inbreeding resulted in increasing rates of miscarriages and infant mortality. The dynasty's final monarch, Charles II, had a jaw so severe his teeth couldn't meet, requiring him to swallow food whole. Despite years of trying, he failed to produce an heir, ending the Spanish Habsburg line.
While most people understood heredity only vaguely, some made remarkable insights. Spanish physician Luis Mercado argued in 1605 that hereditary diseases weren't divine punishments but rather like "stamps" marking seeds that produced similar defects in offspring. He even advised against marriages between people with the same defects-advice the Habsburgs ignored as they continued inbreeding that endangered their dynasty.
The scientific understanding of heredity began with practical breeding. Robert Bakewell revolutionized livestock breeding in 18th-century England by carefully selecting animals with desirable traits. Though he published no formal laws, his success demonstrated heredity's malleability. Following his methods, the Moravian Sheep Breeders' Society sought to discover heredity's underlying principles, with Count Imre Festetics observing that traits could disappear in one generation only to reappear later-a phenomenon we now recognize as recessive inheritance.
The true breakthrough came when Augustinian monk Gregor Mendel conducted meticulous experiments with peas in the 1850s and 1860s. By tracking traits across generations, he discovered consistent patterns-including the famous three-to-one ratio when certain traits reappeared. Mendel proposed that plants contained "antagonistic elements" passed to offspring in pairs, with some elements dominant over others. Though initially overlooked, his work would eventually transform our understanding of heredity.
Chapter 3
From Folk Practice to Scientific Principle: The Birth of Genetics
While European scientists formed societies to study heredity, American farmers took a more practical approach. Rather than conducting controlled experiments, they often simply discovered and propagated unusual plants they found. Jesse Hiatt's persistent apple seedling, which he repeatedly cut down only to have it return, eventually produced the now-famous Delicious apple. Luther Burbank, born in 1849, exemplified this American approach, combining natural curiosity with entrepreneurial spirit.
Burbank's breakthrough came when he discovered a rare potato seed ball containing "all the heredity of the variety." From twenty-three seeds, he produced the "Burbank Seedling" potato, which became famous and whose descendant, the Russet Burbank, now dominates Idaho's potato industry and is the only variety McDonald's accepts for its french fries. This success launched his career as America's premier plant breeder.
Burbank was deeply influenced by Charles Darwin's work on variation in domesticated plants and animals. Darwin had recognized that while his theory of natural selection explained species evolution, he hadn't adequately addressed how traits passed from parents to offspring. Drawing on cell theory, Darwin developed "pangenesis" to explain inheritance, proposing that cells throughout the body cast off "minute granules or atoms" called gemmules, which accumulated in sexual organs and combined during fertilization.
While Burbank built his empire following Darwin's principles, the scientific understanding of heredity was evolving rapidly. August Weismann challenged Darwin's pangenesis by cutting tails from white mice across five generations, producing 901 offspring-all with normal tails. This experiment helped establish that acquired characteristics weren't inherited. Weismann proposed instead that germ cells (eggs and sperm) contained hereditary information that remained separate from the rest of the body's cells.
The rediscovery of Mendel's work in 1900 by Hugo de Vries, William Bateson, and Carl Correns revolutionized heredity science. Bateson demonstrated that animals displayed the same inheritance ratios as plants, and even human diseases like alkaptonuria followed Mendel's patterns. He christened this new field "genetics," declaring that "the whole problem of heredity has undergone a complete revolution."
This scientific revolution, however, would soon be twisted to serve darker purposes, as the emerging field of genetics became entangled with the eugenics movement that sought to "improve" the human race through selective breeding.
Chapter 4
The Shadow of Eugenics: Science Distorted by Prejudice
In 1897, eight-year-old Emma Wolverton arrived at the Vineland Training School in New Jersey, an institution for those deemed "feebleminded." Though she showed no physical abnormalities and could perform basic tasks, school staff classified her as "obstinate and destructive" and feebleminded. Despite eventually becoming competent enough that visitors sometimes mistook her for a teacher, psychologist Henry Goddard tested her and classified her with his newly invented term: "moron."
Goddard, who became Vineland's research director in 1906, initially hoped to find ways to improve intelligence. However, after adopting the Simon-Binet intelligence test from Europe, he concluded that feeblemindedness was an incurable hereditary condition. Working with Charles Davenport of the Station for Experimental Evolution, Goddard began tracing the family histories of Vineland students.
Emma Wolverton's family history particularly interested him. His fieldworker Elizabeth Kite traced 480 Wolvertons to a founding father named John Wolverton, finding 143 feebleminded descendants but also respectable professionals in another branch. She discovered that during the Revolutionary War, John had fathered a child with a "feebleminded" tavern girl, creating two divergent family lines. Goddard used this story as the foundation for his 1912 book "The Kallikak Family," renaming Emma as Deborah Kallikak and John as Martin Kallikak.
The book became a bestseller, portraying feeblemindedness as a hereditary punishment for sin. This fame helped spread Goddard's intelligence tests throughout American schools and to Ellis Island, where immigrants were screened. Testing immigrants yielded shocking results-Goddard claimed 79% of Italians, 83% of Jews, and 87% of Russians tested as "feebleminded." When America entered WWI, Goddard helped create intelligence tests for 1.7 million army draftees, with results showing 47% of white soldiers and 89% of black soldiers qualified as "morons."
The Kallikak story took on a life of its own, used as evidence in the 1927 Supreme Court case that approved Carrie Buck's sterilization, leading to a surge in forced sterilizations nationwide. The book was published in Germany in 1914, and Hitler read about the Kallikaks while imprisoned in 1924, shortly before writing Mein Kampf. When the Nazis came to power, they republished "The Kallikak Family" in 1933, using it in educational films about heredity and selective breeding that supported their sterilization and later euthanasia programs.
Only a few critics challenged this narrative. Walter Lippmann attacked it in 1922, calling the claim that Americans' average mental age was fourteen "nonsense." Biologist Thomas Hunt Morgan, working with fruit flies, discovered that heredity was vastly more complex than eugenicists claimed-with many genes influencing single traits and environmental factors playing crucial roles.
The truth about the Kallikaks wasn't revealed until the 1980s, when genealogists David Macdonald and Nancy McAdams discovered that Elizabeth Kite had mistaken two second cousins both named John Wolverton as father and illegitimate son. The supposedly "feebleminded" branch wasn't a horde of monsters but ordinary people who had suffered poverty and abuse. Emma Wolverton herself spent 53 years institutionalized at Vineland, working in the gymnasium, sewing costumes, and reading books. She died at 89 in 1978, having named a cat "Henry" after "a dear, wonderful friend who wrote a book. It's the book what made me famous."
Chapter 5
Beyond Mendel: The Discovery of Phenylketonuria
Nine years before institutionalizing her daughter at Vineland, Pearl Buck gave birth to Caroline in China in 1920. As Carol grew, she failed to develop normally, not walking or speaking when other children did. After consulting numerous specialists, a doctor at the Mayo Clinic finally confirmed that Carol had stopped developing mentally. Though one doctor offered gentle hope, another bluntly advised Pearl to "find a place where she can be happy and leave her there and live your own life."
In 1929, Pearl found the Vineland Training School. To pay for Carol's care, Pearl wrote furiously, completing "The Good Earth," which earned her the Pulitzer Prize. Despite growing fame and winning the Nobel Prize in 1938, Pearl kept Carol's condition private until 1950, when she finally wrote "The Child Who Never Grew" for Ladies' Home Journal. The groundbreaking essay emphasized that intellectual disability had nothing to do with hereditary taint. Ironically, doctors had already diagnosed Carol with a hereditary disorder a decade earlier but never told Pearl.
That disorder was phenylketonuria (PKU), first identified by Norwegian physician Ivar Asbjrn Flling in 1934. Flling discovered that some children with intellectual disabilities excreted phenylpyruvic acid in their urine, which turned green when ferric chloride was added. He determined it was a recessive condition-parents were healthy carriers, and their children developed the condition only when inheriting two defective copies.
During a 1939 visit to Vineland, researcher Lionel Penrose met nineteen-year-old Carol Buck. Recognizing the telltale musty odor, blue eyes, and fair hair characteristic of PKU, Penrose tested her urine, which turned the "wonderful green color" confirming PKU. The Vineland staff, however, never informed Pearl Buck about her daughter's diagnosis.
After World War II, Penrose became the Galton Professor of Eugenics at University College London but worked to transform the position, eventually renaming it "Galton Professor of Human Genetics." In his 1945 inaugural lecture, "Phenylketonuria: A Problem in Eugenics," Penrose used PKU to dismantle eugenic arguments, calculating that approximately 1% of Britain's population carried the PKU gene without symptoms. Eliminating the gene would require sterilizing all these healthy carriers-something "only a lunatic would advocate" to prevent "a handful of harmless imbeciles."
The breakthrough in treating PKU came in 1949 when Mary Jones brought her 17-month-old daughter Sheila to Dr. Horst Bickel, who developed a phenylalanine-free diet with biochemist Louis Wolff. The diet dramatically improved Sheila's condition-she began sitting up, standing, and walking. Scientists later developed better formulas and earlier detection methods, including Willard Centerwall's diaper test and Robert Guthrie's blood test for newborns. These advances transformed PKU from a devastating condition to a manageable one when caught early.
The McGrath sisters became powerful symbols of PKU treatment's potential. Sheila, diagnosed at age one, suffered irreversible brain damage and lived in an institution. Her younger sister Kammy, diagnosed at just three weeks through Centerwall's test and immediately placed on a special diet, developed normally. Their story helped drive mandatory PKU screening legislation, beginning with Massachusetts in 1963.
Chapter 6
The Complex Web of Genetic Inheritance
The human genome contains far more complexity than early geneticists imagined. While Mendel's patterns apply to some traits, many others follow different rules. Microbes don't follow Mendel's Law-they typically have a single chromosome and reproduce by division. They can also inherit genes horizontally from unrelated microbes through various mechanisms, including ringlets of DNA called plasmids that they can pump into other microbes through tubes.
Even among organisms that reproduce sexually, some genes can hijack heredity through "gene drive"-mechanisms that increase their own transmission rates beyond Mendel's expected 50%. First discovered in the 1920s when Sergey Gershenson found flies producing mostly daughters, gene drives can spread through populations like epidemics.
Our understanding of human genetic relationships has also evolved dramatically. Traditional genealogy traces ancestry through surnames and written records, but DNA testing reveals a more complex reality. Comparing my DNA with distant cousins exposes the counterintuitive nature of genetic inheritance-about 25% of fourth cousins share no identical DNA segments at all. With each generation back, our ancestors double, but the DNA we inherit from them shrinks dramatically.
Mathematical models confirm that everyone alive in Europe a thousand years ago who has any living descendants is an ancestor of every European alive today. This universal ancestry extends globally to about five thousand years ago-making every person alive then with living descendants an ancestor of everyone on Earth today.
The concept of race, long thought to have biological foundations, has been thoroughly undermined by genetic science. When I had my genome analyzed at Yale, researchers compared it to two anonymous volunteers from Nigeria and China. Despite representing three continents and what some might call three "races," we shared 1.4 million genetic variants. I shared another 530,000 variants exclusively with the Chinese person and 440,000 exclusively with the Nigerian. In total, 83% of my genetic variants appeared in at least one of their genomes-demonstrating how much more we share than what separates us.
Race isn't a natural feature of the world but a social concept with a specific history. Ancient writers recognized differences between peoples but didn't classify them taxonomically. The modern concept of race emerged during Habsburg Spain, where "Old Christians" distinguished themselves from those with Jewish ancestry through claims of "pure blood."
While some population groups do show higher rates of certain diseases, these patterns aren't always genetic. Richard Cooper's research on hypertension revealed that rural Nigerians and Cameroonians had lower blood pressure than both Black and white Americans, contradicting decades of medical assumptions about racial predisposition. What truly matters is ancestry-the evolutionary journey from early hominins in Africa to humans spreading across continents, which shaped our inherited genomes.
Chapter 7
The Mosaic Nature of Our Bodies
The traditional view of heredity assumes our bodies contain identical copies of the DNA we inherited at conception. However, research reveals we're actually genetic mosaics-collections of cells with slightly different genetic profiles. From a single fertilized egg, we develop approximately 37 trillion cells, each requiring perfect duplication of three billion DNA base pairs with every division. Despite remarkable precision, mistakes happen, creating daughter cells with new mutations.
These mutations can affect few cells or many, depending on when they occur during development. CHILD syndrome strikes early as the embryo divides into left and right sides, creating a half-dark, half-light appearance. Lines of Blaschko form later as epidermal cells stream from the body's midline. The same mutation can even cause different conditions depending on timing: Sturge-Weber syndrome and port-wine stains both involve the same GNAQ gene mutation, but Sturge-Weber occurs when the mutation happens early enough to affect blood vessels, brain, and skin, while later mutations only affect skin cells.
Beyond mosaicism, our bodies also contain cells from other individuals. In 1953, a 25-year-old woman known as Mrs. McK donated blood in Sheffield, England. Testing revealed her blood contained both type A and type O cells. Further investigation showed she had been born a twin, though her brother died of pneumonia at three months old. Mrs. McK had inherited type O genes from her parents, while acquiring her brother's type A stem cells in the womb.
Even more surprising, Diana Bianchi discovered that fetal cells can survive in women's bodies for decades. She found Y chromosomes in women who had given birth to sons years earlier-one woman had her son's cells after 27 years. Research revealed that all women have fetal cells in their bloodstream by 36 weeks of pregnancy, and up to half still carry these cells decades later. These microchimeric cells don't just circulate-they integrate into organs throughout the body, including the brain, heart, lungs, and liver.
The most extreme form of cellular chimerism occurs with transmissible cancers. In 2006, Elizabeth Murchison discovered that the facial tumors decimating Tasmanian devils weren't ordinary cancers arising from mutations in the devils' own cells. Instead, the tumor cells had entirely different chromosome patterns, indicating they came from another animal entirely. The devils were becoming chimeras, with cancer cells from one devil establishing themselves in new hosts through biting. Even more remarkably, canine transmissible venereal tumor has been spreading between dogs for about 11,000 years, making it a cancer lineage that has survived a thousand times longer than a typical dog.
Chapter 8
The Many Dimensions of Human Heredity
The height of my daughters prompted family comparisons that reminded me of my youth when my brother and I outgrew both our parents. Relatives would ask, "Where did you get that from?" as if height were a diamond passed down through generations. But unlike simple genetic conditions like PKU, height's heredity remains puzzlingly complex-simultaneously shaped by genes and environment.
Twin studies by Karri Silventoinen involving over 30,000 twin pairs estimated height's heritability at 70-94% for men and 68-93% for women. Peter Visscher later confirmed these findings through a different approach, examining 11,214 regular sibling pairs with varying genetic similarity, calculating height's heritability at 86%-exceptionally high compared to traits like nicotine dependence (60%), menopause timing (47%), or left-handedness (26%).
Despite height's strong heritability, environment profoundly shapes stature. Louis-Rene Villerme observed French soldiers' height declining during Napoleonic War food shortages, then rebounding afterward. European men from the Gravettian culture stood six feet tall 30,000 years ago, but lost eight inches after adopting agriculture 8,000 years ago, likely due to less protein-rich diets. Heights remained relatively stable until the late 19th century, when a dramatic increase began-Americans grew three inches before plateauing in the 1990s, while Europeans continue growing about half an inch per decade.
Joel Hirschhorn sought to understand height variation beyond rare conditions like Laron syndrome. His team discovered that many height-related genes are particularly active in growth plates-thin layers of cells near bone ends where cartilage cells multiply to lengthen bones. In 2017, Jonathan Pritchard's team reanalyzed Hirschhorn's data and made a startling discovery: at nearly two million spots across the entire human genome, they detected tiny influences on height. Each variant had a minuscule effect, but collectively these weak variants explained more height variation than the strongest genes in Hirschhorn's catalog.
Intelligence shows similar complexity. Early intelligence researchers like Charles Spearman claimed intelligence was entirely hereditary, but lacked scientific evidence. Modern twin studies consistently show intelligence heritability around 50%. Unlike Galton's belief that intelligence was purely hereditary, modern science shows intelligence isn't fixed like blood type. James Feyrer's research revealed how introducing iodized salt between the World Wars dramatically impacted intelligence-raising average IQ by 3.5 points nationwide, and up to 15 points in iodine-deficient regions.
Crucially, socioeconomic status affects heritability itself. In affluent families, intelligence heritability reaches 60%, but approaches zero in poor families-poverty can overwhelm genetic influences. Paradoxically, heritability increases with age (42% at nine, 68% at seventeen) as children increasingly shape their own environments based on their genetic predispositions.
Chapter 9
Redefining Heredity for the Future
How does a single cell develop into a complex human body with trillions of cells? This question has puzzled scientists for centuries, revealing that heredity operates in multiple forms within our own bodies. Each of our 37 trillion cells sits on a branch of a genealogical tree extending back to conception. These specialized cells form separate branches on the body's internal genealogical tree, like rival dynasties descended from a single monarch.
Beyond our own cells, we inherit vast microbial communities. When researcher Rob Dunn analyzed my belly button bacteria, he found fifty-three different species, seventeen unique to me. Our microbiomes have co-evolved with us for over fifteen million years, with bacterial lineages branching in patterns that match our evolutionary relationships with other primates. Twin studies reveal that our genes help determine which microbes we harbor, with some bacterial species showing heritability comparable to anxiety.
The possibility that epigenetic changes could pass from one generation to the next sparked excitement about a new kind of heredity. Studies suggesting that chemicals like vinclozolin and DEET could have effects across generations raised concerns about environmental pollutants creating multi-generational legacies. However, many scientists remain skeptical about transgenerational epigenetic inheritance in mammals, pointing to biological barriers that should prevent it. During fertilization and early embryonic development, most epigenetic marks are stripped away and reset, creating what Kevin Mitchell called "radical amnesia in every generation."
Cultural heredity represents yet another inheritance system. When my daughter Charlotte participated in experiments comparing human and chimpanzee learning styles, she faithfully reproduced every demonstrated action, even useless ones-evidence of humans' deep adaptation for cultural inheritance. We pass down not just genes but recipes, songs, knowledge, and rituals essential for survival. Aboriginal Australians inherited not just genes and culture, but also human-altered environments. Our ancestors began altering their surroundings with fire a million years ago, initially for cooking, then for tool-making and landscape modification.
The advent of CRISPR technology has given us unprecedented power to alter heredity directly. Jennifer Doudna, who helped discover CRISPR's gene-editing capabilities, has expressed deep concerns about its potential misuse. The technology offers tremendous potential benefits: malaria-resistant mosquitoes could save hundreds of thousands of lives yearly; mice could be engineered to resist Lyme disease; herbicide-resistant weeds could be re-sensitized. However, the National Academy of Sciences warned that gene drives could cause "irreversible effects on organisms and ecosystems"-potentially altering not just genetic inheritance but ecological inheritance for generations to come.
To address problems like global warming and inequality, we need what Zimmer calls "a social form of CRISPR"-a way to alter the practices and values transmitted between generations. Despite cynicism, environmental scientist Erle Ellis points to cultures that have successfully transmitted customs allowing them to thrive without environmental destruction. When considering technologies like CRISPR, we must ensure they truly rework the world as needed, recognizing that heredity extends far beyond the genes we pass to our children-it encompasses the entire world we leave behind.
