Genes are biochemical recipes written in a four-letter alphabet called DNA, recipes for how to make and run a body. A normal human being has two copies of each of 30,000 genes in every cell in his or her body. The total complement of 60,000 human genes is called the “genome,” and the genes live on twenty-three pairs of ribbonlike objects called “chromosomes.” When a man impregnates a woman, each one of his sperm contains one copy of each gene, 30,000 in all, on twenty-three chromosomes. These are added to the 30,000 single genes on twenty-three chromosomes in the woman’s egg to make a complete
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