Is the underarm 'shadow' heritable?

Ladies, when you shave your underarms, do you still have a poorly lit spot that never seems to walk away? My sister also has the problem and I didn't know if it's newly something I have because I'm built that path, or another reason. If anyone have found a solution for this, help would be appreciated, thankfulness. It's like that because you do shave. It doesn't issue if the razor is clean or not. Some people merely have this "side effect" from shaving sensitive areas of their bodies.
conceivably you need a better cut-throat. i had matching problem until i got the Venus Breeze, and in a minute i never have that problem. (: