Genome

Science writer Matt Ridley organizes this nonfiction exploration of the human genome around a single conceit: One gene selected from each of the 23 human chromosome pairs illuminates a different theme of human nature. Written as the Human Genome Project, the international effort to sequence the complete set of human genes, raced toward completion, the book treats the genome as an autobiography of the human species, a document recording evolutionary history in digital code. Ridley frames genetic knowledge as a blessing rather than a curse, though he acknowledges dangers such as insurance discrimination and biological warfare.

A primer section establishes foundational vocabulary. The genome is compared to a book: Its 23 chromosome pairs are chapters, its genes are stories, its exons (coding segments) are paragraphs interrupted by introns (non-coding stretches), and its four chemical bases, A, C, G, and T, are letters. DNA copies itself through base-pairing, gets transcribed into RNA, and is translated into proteins by molecular machines called ribosomes. Ridley traces life's origin through chromosome 1, arguing that RNA preceded both DNA and protein as the first self-replicating molecule, and that the universality of the genetic code across all organisms demonstrates that all life descended from a single origin.

Chromosome 2 provides a window into human evolution. Humans have 23 chromosome pairs while other great apes have 24, because two ancestral chromosomes fused. DNA differs by less than two percent between humans and chimpanzees. Ridley traces the human lineage through bipedalism, bare skin for thermoregulation, meat-eating to fuel expanding brains, and a uniquely human sexual division of labor that promoted monogamy and food sharing. The history of genetics occupies chromosome 3, from physician Archibald Garrod's 1902 hypothesis that genes are recipes for enzymes, through Gregor Mendel's discovery of particulate inheritance, to James Watson and Francis Crick's 1953 discovery of DNA's double-helix structure. The genetic code was fully deciphered by 1965.

Huntington's disease, explored through chromosome 4, represents the extreme case of genetic determinism. A repeating three-letter sequence, CAG, precisely predicts the age of onset: 39 repeats yield symptoms around age 66, while 50 repeats yield symptoms around age 27. Ridley recounts how researcher Nancy Wexler, whose mother had the disease, traveled to Venezuelan villages to study 11,000 descendants carrying the mutation, helping locate the gene in 1993. Yet only about 20 percent of those at risk choose to take the test, since knowing the cause has not produced a cure.

Ridley then pivots to complexity. Asthma, discussed through chromosome 5, illustrates the messy reality of most genetic influence: by 1998, scientists had found 15 candidate genes, none explaining more than a small fraction of cases. The chapter on chromosome 6 tackles intelligence. Twin studies show about half of IQ variation is heritable, yet the correlation of IQ between adopted children reared together is zero, suggesting shared family environment has little lasting effect. Ridley stresses that heritability of individual differences does not imply heritability of group differences, and notes that psychologist Robert Plomin's gene on chromosome 6 adds only about four IQ points on average.

Language provides the focus for chromosome 7. Drawing on linguist Noam Chomsky's theory of universal grammar, Ridley presents evidence that grammar is innate: Children impose grammatical rules on pidgin languages (simplified contact languages lacking fixed grammar), deaf Nicaraguan children spontaneously invented a sign language with full grammar, and specific language impairment, linked to a gene on chromosome 7, disrupts grammatical ability without affecting general intelligence. The X and Y sex chromosomes reveal evolutionary conflict between the sexes: The X chromosome evolves genes that attack the Y, which has responded by shedding most of its genes. Evolutionary biologist David Haig's theory reinterprets the placenta as a paternal organ that parasitizes the mother's blood supply, illustrating how genes from each parent can conflict within the same fetus.

Chromosome 8 reveals that 97 percent of the genome consists not of true genes but of "selfish" DNA, parasitic sequences that persist because they are good at getting themselves copied. The practical application of this repetitive DNA came through geneticist Alec Jeffreys's 1984 discovery of DNA fingerprinting. Blood groups on chromosome 9 demonstrate how infectious disease maintains genetic diversity: People with AB blood are virtually immune to cholera while those with type O are most susceptible, creating frequency-dependent selection, a dynamic in which the rarest variant has a survival advantage that keeps multiple versions of the gene in circulation.

Chromosome 10 dismantles mind-body dualism. The CYP17 gene encodes an enzyme that converts cholesterol into cortisol, the stress hormone, which suppresses the immune system. The crucial insight is circular causality: Behavior alters hormones, which alter gene expression, which alters behavior. The Whitehall study of 17,000 British civil servants found that job status predicted heart attack risk more strongly than obesity, smoking, or high blood pressure. Personality genetics on chromosome 11 reinforces this point: The D4DR gene, encoding a dopamine receptor, correlates with novelty-seeking behavior but explains only about four percent of the variation. Ridley estimates roughly 500 genes may contribute to personality.

Embryonic development, explored through chromosome 12, involves Hox genes, master regulatory genes found in clusters arranged in the same order as the body parts they control. This developmental mechanism has been conserved for over 530 million years, from fruit flies to humans. Chromosome 13 links genetics and linguistics: Geneticist Luigi Luca Cavalli-Sforza uncovered genetic gradients across Europe matching the archaeological record of farming's spread. Chromosome 14 addresses aging through telomeres, repeated sequences at chromosome ends that shorten with each cell division. The evolutionary theory of aging holds that natural selection cannot eliminate genes that damage the body after reproduction ends.

Genetic imprinting on chromosome 15, the phenomenon in which genes behave differently depending on which parent they came from, reveals conflict between paternal and maternal genomes. Prader-Willi syndrome and Angelman syndrome both result from the same chromosomal deletion, but the first occurs when the father's copy is lost and the second when the mother's copy is lost. Chromosome 16 explores memory: The CREB protein switches on genes that alter synapses, and animals lacking it can learn but cannot retain memories beyond an hour. Cancer occupies chromosome 17: The TP53 gene encodes p53, which detects DNA damage and instructs cells to halt division or undergo apoptosis, or programmed cell death. TP53 is mutated in 55 percent of all human cancers.

Chromosome 18 covers genetic engineering, from the first recombinant DNA in 1972 through the 1990 treatment of Ashanthi DeSilva, a three-year-old with severe combined immune deficiency who received genetically engineered copies of the gene her immune system lacked. Chromosome 19 raises the ethics of predictive medicine through the APOE gene, whose E4 variant increases risk of both heart disease and Alzheimer's disease; Ridley argues that individuals should control their own genetic information. Prion diseases on chromosome 20 reveal a form of self-replication outside normal genetics: The PRP gene produces a brain protein that can misfold into a lethal form capable of converting normal copies to its shape. Ridley traces these diseases from scrapie in sheep through kuru, spread by funereal cannibalism among the Fore people of Papua New Guinea, to the BSE (mad cow disease) epidemic in British cattle.

The history of eugenics occupies chromosome 21, chosen because Down syndrome, caused by an extra copy of this chromosome, is the most common target of genetic screening. Eugenic ideology led to the sterilization of over 100,000 Americans and 400,000 Germans. Ridley credits libertarian MP Josiah Wedgwood with preventing similar laws in Britain through his 1912 opposition to the Mental Deficiency Bill, and concludes that the problem with eugenics was never the science but the coercion.

The final chapter, on chromosome 22, addresses free will. Ridley argues that genetic determinism is no more threatening to freedom than environmental determinism, noting that Freudian, Marxist, and behaviorist explanations of human behavior are equally deterministic. Drawing on psychologist Judith Rich Harris's research, he notes that studies controlling for heritability find no evidence that parenting shapes personality outside the home. Ridley resolves the paradox by arguing that freedom lies in expressing one's own nature rather than being shaped by external forces: A genome unique to each individual is the strongest bulwark against outside control.