Chapter 19, Problem 25P

In human gestational development, abnormalities of the closure of the lower part of the midface can result in cleft lip, if the lip alone is affected by the closure defect, or in cleft lip and palate (the roof of the mouth), if the closure defect is more extensive. Cleft lip and cleft lip with cleft palate are multifactorial disorders that are threshold traits. A family with a history of either condition has a significantly increased chance of a recurrence of midface cleft disorder in comparison with families without such a history. However, the recurrence risk of a midface cleft disorder is higher in families with a history of cleft lip with cleft palate than in families with a history of cleft lip alone.

a. Suppose a friend of yours who has not taken genetics asks you to explain these observations. Construct a genetic explanation for the increased recurrence risk of midface clefting in families that have a history of cleft disorders versus families without a history of such disorders.

b. Construct a similar explanation of why the recurrence risk of a cleft disorder is higher in families with a history of cleft lip with cleft palate than in families with a history of cleft lip alone.