Topic: MHS2WOR HR Endgame questions

So, I just got into the high rank post game and now there are the opportunities to get "high rank" monsters and dens. Obviously all of the monsters I built in the low rank game basically are useless right now, but I'm just wondering, what decides if a monster is HR or LR, because looking at them from the stables the only determining factor seems to be their gene makeup. There is no indicator as to whether or not a monster is HR or LR and if it is only based on gene makeup, does that mean a LR monstie can be converted to HR if you give it all XL genes? (XL genes seem to be the only factor I can determine so far that designates a monstie as high rank). Thanks for the info!