Cystic fibrosis is a recessive trait, which means that the disease phenotype is caused by inheriting two recessive alleles, as in Emily’s case. Emily’s parents do not have CF because they each possess one dominant allele, but they each also carry one recessive CF allele, making them both heterozygous carriers. To calculate the probability that Debbie and Lowell will have a child with CF, we can determine the possible alleles in their gametes and then join all possible combinations of these sperm and egg in a Punnett square.