The book devotes one chapter to each pair of human chromosomes. Since one (unnumbered) chapter is required to discuss the sex chromosomes, the final chapter is number 22. Ridley was inspired to adopt this model by Primo Levi's book The Periodic Table.[6]
- Chapter 1, Life
The first chapter begins with a quote from Alexander Pope on the cycle of life. The very broad topic "Life" is also the topic of the chapter. Ridley discusses the history of the gene briefly, including our "last universal common ancestor".
- Chapter 2, Species
Ridley discusses the history of human kind as a genetically distinct species. He compares the human genome to chimpanzees, and ancestral primates. He also points out that until the 19th century, most scholars believed that there were 24 sets of genes, not 23 as known today.
- Chapter 3, History
This chapter discusses the interplay between early geneticists, including Gregor Mendel, Charles Darwin, Hermann Joseph Muller and Francis Crick.
- Chapter 4, Fate
Huntington's chorea is used to discuss the use of a particular sequence on Chromosome Four to cause traumatic health consequences. The search for the chromosomal source of this and other related diseases is discussed through the work of Nancy Wexler, someone who may have inherited the gene but who turns to scientific work to study it in others.
- Chapter 5, Environment
The concepts of pleiotropy and genetic pluralism are introduced. A brief history of the study of asthma is used as the case study. Asthma is related to as many as fifteen different genes, many on chromosome five. Specifically, this includes a change from adenosine (A) to guanine (G) at position 46 on the ADRB2 gene. The ADRB2 gene is related to the control of bronchodilation and bronchoconstriction.
- Chapter 6, Intelligence
Robert Plomin's announcement in 1997 of the discovery of "a gene for intelligence" on chromosome 6 is the foundation for this chapter's lengthier discussion of the genetic basis for intelligence. This included gene IGF2R on the long arm of chromosome 6, which may also be related to liver cancer. Ridley continues his premise in this chapter that the use of simple genetic markers is inadequate to describe the complete function of the genome, or the causation of disease.
- Chapter 7, Instinct
This chapter discusses whether the form and existence of language has a genetic component. In particular, "specific language impairment" is possibly related to a gene on chromosome 7. Ridley discusses the scientific disagreement between Canadian linguist Myrna Gopnik and others on whether this disorder relates to difficulties with grammar formulation, or is a broader intellectual disorder.
- Chapter X and Y, Conflict
Ridley contemplates evolutionary psychology using the genes SRY on the Y chromosome, and DAX1 and Xq28 on the X chromosome. The theory of genetic conflict and evolution is debated using the rhetorical question, are we bodies containing genes, or genes in bodies?
- Chapter 8, Self-Interest
Richard Dawkins's concept of the "selfish gene" is described by Ridley through a discussion of retrotransposons. This includes the behavior of the LINE-1 and Alu transposons. Further, Ridley discusses the possible purposes of cytosine methylation in development. The chapter also discusses how, through reverse transcriptase, retroviruses like HIV copy themselves to the human genome.
- Chapter 9, Disease
For chromosome 9, the book examines the discussion of the blood-typing genetic sequences. Namely, the ABO blood groups and their impact on evolution are discussed. Other genes mentioned include CFTR for cystic fibrosis. Ridley concludes that the Human Genome Project is largely based on the inaccurate belief that there is one single human genome. Proof that this is wrong comes from answering the question, which of the several choices of blood typing genetic sequence is selected, since each one has different disease-resistant and evolutionary consequences?
- Chapter 10, Stress
The impact of stress on the human body is described starting with the creation of hormones by the CYP17 gene on chromosome 10. Ridley points out the relationship between cholesterol, steroidal hormones such as progesterone, cortisol, aldosterone, testosterone and oestradiol.
- Chapter 11, Personality
Ridley chooses the gene D4DR which codes for the manufacture of dopamine and is located on the short arm of chromosome 11. Interactions between dopamine, serotonin and other serotonin neurochemistry are lightly covered.
- Chapter 12, Self-Assembly
This chapter relates to how understanding the genetic code matches models for embryonic development among vertebrates. Ridley discusses 'gap' genes, 'pair-rule' genes, and 'segment-polarity' genes. Homeotic genes and Hox genes are described briefly. Walter Gehring's discovery of the homeobox set of codes in 1983 is related to an on and off switch metaphorically.
- Chapter 13, Pre-History
Ridley describes the relationship between the development of Indo-European and other ancient root languages and the classical polymorphisms which map genetic frequencies in Eurasia. The interplay between the breast cancer genes BRCA2 on chromosome 13 and BRCA1 on chromosome 17 help to illustrate these larger concepts. Ridley also describes genetic studies of different types of peoples to isolate why people developed a mutation allowing adults to digest lactase in adulthood. He concludes that since the herding tribes of the world all evolved this mutation earliest, these people's genes adapted to their environment. This may sound like Lamarck's tale of the blacksmith's strong arms 'handed down' to his sons, but it is not. The controversial conclusion is that willed action can alter our evolutionary history and genetic composition, by changing the environment to which we have to adapt.
- Chapter 14, Immortality
This chapter examines the so-called "immortality" of the genetic code - i.e. how is it that genetic code can remain as precise as it has been for 50 billion copyings since the dawn of life? Part of the answer is in the protein enzyme telomerase, lying on chromosome 14 and coded by the gene TEP1.
- Chapter 15, Sex
Ridley discusses two chromosome 15 genetic diseases, Prader-Willi syndrome and Angelman's syndrome: Prader-Willi inherited from the father, Angelman's from the mother, through sexual antagonism and the placenta's control by paternal genes.
- Chapter 16, Memory
Ridley debates the old knowledge versus instinct problem, claiming that natural selection will be the explanation of the instinct for grammar, and noting that many animals including invertebrates can learn. All the same, he argues that the brain is controlled by genes and gene products.
- Chapter 17, Death
The TP53 gene on chromosome 17 suppresses cancer cells, while oncogenes stimulate cell growth and can cause cancer if kept switched on, while TP53 can cause cancer when kept switched off. Other mutator genes like BRCA1 and BRCA2 contribute to breast cancer.
- Chapter 18, Cures
Recombinant DNA enabled genetic manipulation with restriction enzymes and a ligase. Genetic engineering has been highly controversial, especially in food production; it might, writes Ridley, one day be used in humans.
- Chapter 19, Prevention
It might be possible to prevent or cure Alzheimer's disease and coronary heart disease. APO genes like APOE influence fat and cholesterol metabolism. The E4 allele of EPOE contributes to the plaque buildup of Alzheimer's. Genetic testing may help patients take early preventative action.
- Chapter 20, Politics
The sheep brain disease scrapie appeared to be infectious but did not involve a microorganism. The disaster of Creutzfeldt–Jakob disease in humans was found to be caused by the PRP gene which produces a prion protein that aggregates into clumps, destroying brain cells. Ridley attacks the panicky handling of prion disease outbreaks by governments.
- Chapter 21, Eugenics
Eugenics a century ago, based on faulty knowledge of genetics, led to immoral actions by governments and the US Supreme Court, pushing through compulsory sterilization of people such as those with trisomy 21 which causes Down syndrome. Ridley discusses the conflict between society, in the form of the state, and the individual.
- Chapter 22, Free Will
Ridley addresses the heated debate between genetic determinism and freedom. Children are moulded both by their peers (other children) and by their genes. He argues that behaviour is in the short term unpredictable, but "broadly" predictable in the long term.
