Question

Which of the following calculations require that you utilize the addition rule?

Answer 1

The answer is D) Calculate the probability of a child having either sickle-cell anemia or cystic fibrosis if parents are each heterozygous for both. 

The addition rule is used to calculate the probability of one of the events from multiple pathways. If you want that only one of the events happens, you will use the addition rule. In a case when you want to calculate the possibility of a child having EITHER sickle-cell anemia OR cystic fibrosis, you will use THE ADDITION RULE.
For example, in statement B) you will not use this rule because there is no either/or, but both/and. In case, B) you would use the multiplication rule.