I have a little cat, she is about 3 years old but she was the runt of her litter. She lives in the house and doesn't have fleas or ticks or mange or anything like that, but the hair on her lower belly is getting thinner and thinner. She's not sick or anything, just losing hair/thinning hair, and it's strange. What might this be from?

Her belly was covered in white fur but now I can see the pink and gray splotched skin underneath. She's brown and black striped/tabby with a white belly, white chin, white chest patch, white toes, could it have to do with genes or something?

I just am concerned, I love that little kitty so much!