three X chromosomes
... that one in a thousand women have three X chromosomes, but fewer than 10 percent of them know it?
... that one in a thousand women have <b id="mwA_s"><a href="./Trisomy_X" id="mwA_w" rel="mw:WikiLink" title="Trisomy X">three X<span id="mwA_0" typeof="mw:Entity"> </span>chromosomes</a></b>, but fewer than 10 percent of them know it?