Only as metaphor may one speak of “black genes” and “white genes,” or of “white” and “black” blood. But once invoked, the metaphor launches a logical program of its own: If “blood” is synonymous with “race,” and “DNA” is synonymous with “blood,” then “DNA” is synonymous with “race.” Although spurious, that synonymy engages a powerful logic in its turn. Invoke a race, and the notion of a distinguishing blood stands to reason. In the folk lexicon, that is precisely what race means.
