What is alopecia, which affects Jada Pinkett Smith, and did the joke go too far?

NEW YORK -- The so-called Slap Heard Round the World happened when Will Smith slugged Chris Rock after Rock made a joke about the close cropped hair of Smith's wife, Jada Pinkett Smith. She's chosen to keep her hair short because she has a medical condition called alopecia, which affects hair growth. She's by no [...]