It is common to talk about traits being monogenic or polygenic. We say that Sickle-cell disease is monogenic because its heritable variation among humans can be accounted for by a single locus of genetic variation. Or more accurately, we say that 100% of the heritable variation can be accounted for by variation in that genetic locus (assuming the simple monogenic scenario, I did not look into the details). We say that height is strongly or highly polygenic because it seems like we need thousands of locuses of genetic variation to account for the heritable variation among humans. The largest study that I know of,