The Ledger of Good Genes

Lazarus Long's godson wants to marry his own sister, and the book solves this not with law but with arithmetic. Two hundred ova, five carrying a lethal recessive; two hundred spermatozoa, five carrying the same. Run the Mendelian distribution across a generation, convert the ugly numbers into percentages, and you get 95.18 percent clean — better odds, the text insists, than a union of unrelated strangers. "Y'r durn tootin' it's 'not always,'" Lazarus tells his computer, dismantling the incest taboo the way you'd audit a bad spreadsheet. Minerva supplies the definition that makes it possible: "'Incest' is a legal term, not a biological one." The taboo dissolves not because anyone argues against it but because a sufficiently good genetic accountant renders it irrelevant.

That scene is worth sitting with because it has become, unglamorously, a business model. Embryo-screening startups now sell exactly this kind of culling as a consumer product — polygenic risk scores, carrier-status charts, the