The validity of punnet squares in determining the paternity for a child

They are designed to predict the probability of some outcome.
  In relation to punnette squares, it's a good way of understanding how biologist (geneticists) can predict the likelihood of inheriting particular traits.
 This also gives parents the predicted frequency of all of the potential genotypes among the offspring each time reproduction occurs. How? Because a parent might want to know the likelihood of a child bearing a certain genetically inherited disease such as cystic fibrosis. 

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