that last pix if it's the one looseing hair the skin looks a lil to red to be hairless i'd say it is some kind of disease reason being hairless mice don't start with hair then loose it they never grow any hair now if your mice never would of got hair in the first place then i'd say you found a hairless gene. what are your temps in there cages i'm sure mice are just like any other hair animal as in when it gets hotter they molt some of there hair to coop with the heat if they can't find someplace cooler. or it could be with the temps they aint growing hair in as quickly