No hair

Question
I recently got a one year old yorkie from a friend who has hair loss/no hair on his spine back to his tail.  There is some hair there but it is not a lot.  What could this be?  I've known the yorkie since before I got him and he's always been like this.  Thank you for your time!

Answer
IF the hair is gone then there are two possibilities.  He could have been born with a defect in the hair follicles there.  Or there could have been a skin injury, like a burn, that affected the hair follicles so the hair won't grow back.   Severe flea allergy could cause this, too.