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The passage around this formula

Set s = 1 , the theoretical maximum in which every affected embryo is detected and never implanted: for q = 0.01 , this gives $q_{n+1}$ = $q_n$/(1+$q_n$) $\approx$ 0.009901 , a reduction of under one-tenth of one percent of the allele’s frequency in a single generation, even at total, universal, perfectly effective screening. The reason the effect is so small is structural, not a modeling artifact: for a rare recessive allele, the overwhelming majority of copies are carried silently in heterozygous carriers, invisible to any selection that acts only on homozygotes, and embryo selection — like natural selection against recessive disease before it — can only ever act on the small fraction of matings…

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