Naomi and her sister have the same allele for the gene of interest; however, Naomi has cleft lip, whereas her sister has only lip pits. What genetic process explains this difference?

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The fact that two children in the same family have lip abnormalities strongly suggests a genetic cause. In this case, a gene mutation responsible for lip abnormalities is found and is autosomal dominant in transmission, with a reduced penetrance of 80% to 90%. (Recall from Chapter 4 that 80% to 90% penetrance means that out of 100 people who have the gene variant that causes the disease, between 10 and 20 of them will show no clinical signs.) For the mutation in this case, about 70% of people with lip pits also have cleft lip (CL) or cleft palate (CP). Some family members with the same gene variant can have only lip pits, whereas others have CL/CP, and still others have just CP. This is an example of the variable expressivity.

Naomi and Her Sister Have the Same Allele for the Gene