The second part of this preface is intended as a brief primer, a sort of narrative glossary, on the subject of genes and how they work. I hope that readers will glance through it at the outset and return to it at intervals if they come across technical terms that are not explained. Modern genetics is a formidable thicket of jargon. I have tried hard to use the bare minimum of technical terms in this book, but some are unavoidable.

The human body contains approximately 100 trillion (million million) CELLS, most of which are less than a tenth of a millimetre across. Inside each cell there is a black blob called a NUCLEUS. Inside the nucleus are two complete sets of the human GENOME (except in egg cells and sperm cells, which have one copy each, and red blood cells, which have none). One set of the genome came from the mother and one from the father. In principle, each set includes the same 60,000-80,000 GENES on the same twenty-three CHROMOSOMES. In practice, there are often small and subtle differences between the paternal and maternal versions of each gene, differences that account for blue eyes or brown, for example. When we breed, we pass on one complete set, but only after swapping bits of the paternal and maternal chromosomes in a procedure known as RECOMBINATION.

Imagine that the genome is a book.

There are twenty-three chapters, called CHROMOSOMES.

Each chapter contains several thousand stories, called GENES.

Each story is made up of paragraphs, called EXONS, which are interrupted by advertisements called INTRONS. Each paragraph is made up of words, called CODONS. Each word is written in letters called BASES.

There are one billion words in the book, which makes it longer than 5,000 volumes the size of this one, or as long as 800 Bibles. If I read the genome out to you at the rate of one word per second for eight hours a day, it would take me a century. If I wrote out the human genome, one letter per millimetre, my text would be as long as the River Danube. This is a gigantic document, an immense book, a recipe of extravagant length, and it all fits inside the microscopic nucleus of a tiny cell that fits easily upon the head of a pin.

The idea of the genome as a book is not, strictly speaking, even a metaphor. It is literally true. A book is a piece of digital information,

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