Two brown-haired people have two children, P and Q. P has blond hair. We therefore believe that each parent is heterozygous and that blond hair is a recessive trait. Q has brown hair. What is the probability that Q is heterozygous?
My answer: 2/3. There are four possibilities Bb, bB, BB,and bb. Before observing the color of Q's hair, we assign equal chance to each. We eliminate bb as a possibility. The chance that it is bB or Bb and not BB is 2/3.
A Biology PhD disagrees with me and says that the probability is 1/2. Who is right?