Also I would have no confidence in estimates of the population that don't involve either a systematic search, or a symptom we're unavoidably certain we'd spot.

Humans who are 10 feet tall would stand out. Humans with XXX were unheard of until they found one by accident and then it turns out they weren't that rare but we'd never gone out of our way to look.

(Having the wrong number of copies of most chromosomes causes something obvious and often fatal, but humans necessarily have 1 or 2 copies of X anyway and cope so 3 isn't that astonishing)

no, because the original clause was talking about the number of people. the joke was that it was identical to the percentage. Extremely circuitous, but meaningful!