An English scientist named Reignald Crundall Punnett developed a useful model called the Punnett square that can be used to predict the probability of parents passing on their allele to their offspring. The square below shows an example from Family 2 parents 1 and 2, the father or sperm parent on the top and the mother or egg parent on the left side.
We know that the father does not have a p53 mutation, so both of his alleles are version A. The mother does have cancer caused by a p53 mutation, so we know she has at least one version of p53C. Since we are not sure what her second allele is, we mark it with a question mark.