Chance events—injuries, infections, infatuations; the haunting trill of that particular nocturne; the smell of that particular madeleine in Paris—impinge on one twin and not the other. Genes are turned “on” and “off” in response to these events, and epigenetic marks are gradually layered above genes.3 Every genome acquires its own wounds, calluses, and freckles—but these wounds and calluses “exist” only because they have been written into genes. Even the environment signals its presence through the genome. If “nurture” exists, it is only by virtue of its reflection in “nature.” That idea
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