XYy Yyy XY Yy YO y | XXy Xyy yyy Xy yy X y O ---+-------------------------------- XY | XXXY XXYy XYyy XXY XYy XYO Yy | XXYy XYyy XXY XYy XYO Yy | XXYy XYyy XXY XYy Yyy XY Yy YO y | XXy Xyy XX Xy yy yO O | XXO XYO yyO XO yO OO Since at least two levels of expression (many have more), called alleles, which