This book has 23 chapters (22 titled chapters, plus a discussion of 'x' and 'y' chromosomes) because the human genome has 23 pairs of chromosomes.
In the first chapter, Ridley teaches about life itself, what life is technically, what evolution has mean to the development of species, and how life is shaped by DNA. Then in chapter 2, he discusses species of animals more specifically, looking at primates as a case study, and explaining the common genetic ancestry of chimpanzees and humans.
Chapter 3 discusses the history of genetic studies, from Darwin's contribution to the scientific discoveries of Crick, Mendel and friends. Chapter 4 discusses the likelihood of inherited health problems (since DNA mutation is responsible for many health issues). Then, in chapter 5, a case study is examined, showing how pleiotropy and pluralism contribute to health issues like asthma.
Chapter 6 discusses the scientific discover of a gene for intelligence on chromosome 6. However, the discovery is not plain or simple, and Ridley takes this opportunity to explain why finding genes like this isn't necessarily helpful, just because genes are way more complex than simply finding genes that mean one thing or another. For instance, this intelligence gene also has some ramifications for liver health.
Instinct is the focus of chapter 7, especially instincts relating to communication, language, and grammar. Then comes the dilemma of 'x' and 'y' chromosomes which determine gender. Chapter 8 explores Richard Dawkins's idea of "selfish genes." The issues of transcription are raised, including an illustration of how HIV infects the human genome through "reverse transcriptase."
Then Ridley discusses diseases like cystic fibrosis. By the end of chapter 9, Ridley argues that the famous Human Genome Project is misguided, since that community believes in a universal human genome. Chapter 10 is all about hormone experience: testosterone, progesterone, cortisol and many more are discussed. Chapter 11 explains the genetic origins of personality.
Chapters 12 and 13 discuss the mechanisms of genetic code development, especially showing that certain situations can produce certain changes in the flow of the gene pool. He explains this by examining the genetic mutation that allowed early Indo-European peoples to digest lactase. These conversations end up in chapter 14 when Ridley discusses the idea of immortal life. Why does life still work after nearly 50 billion reiterations? Ridley concludes that the secret to DNA's success on earth comes from telomerase on the 14th chromosome.
Sex is the subject of chapter 15. The diseases of Prader-Willi syndrome and Angelman's syndrome are used to show how gendered genes can pass along health issues. In chapter 16, Ridley discusses the genetic underpinning of memory and knowledge. His opinion is simply that the brain functions according to DNA's instruction and design. In 17, Ridley discusses death. Certain genes (when not deactivated) can lead to the development of cancers. This leads to 18, where Ridley explains his forecast that genetic engineering will be a real part of human life in the future, because of its potential medical uses. In 19, Ridley explains that by examining certain genetic codes, we might be able to alert people who are at greater risks for certain illnesses, especially Alzheimer's disease.
In chapter 20, Ridley shows how politicians who don't understand genetics are at risk of responding badly to disease outbreaks, like the sheep brain disease, 'scrapie.' 21 is about Eugenics itself. In the past, immoral actions were taken by the US Supreme Court in the past, like pushing for compulsory sterilization of carriers of Down Syndrome.
This leads to a discussion of free will in chapter 22. By practicing genetic determinism, freedom might be at risk, but Ridley explores both sides of the debate, reminding the reader that humans are shaped by nature and nurture both.
