Well as I understand it loci are more like ranges not on-off switches, like every single variable (traits) like length of nose, width, colour of eyes, hair colour, width of cheekbones, prognatism of jaw, and so on...    for example eye colour can be a continuous range with all the shades from pitch black to silver blond....    races are just the clustering of certain sub-ranges for every single trait.    you can always have extremes that overlap, like a white person with thick lips (or thicker lips than the average for white) that could probably be as thick or thicker than a negro that's in the lowest value in the range or even maybe the negro's average but you won't find a white with lips as think as the highest values in the negro range...    of course these ranges are a normal curve where the average non-overlaping values are the more common, but you can always have some small percentage of people in the extremes....