Question 3 Item 3
The figure presents a pedigree of the incidence of protoporphyria in four generations of a particular family. An affected male and an unaffected female represent the first generation. They produce six second-generation children, including four affected females and two unaffected males. One of the unaffected second-generation males has children with an affected female. They produce five third-generation children, including an affected male, an affected female, two unaffected males, and an unaffected female. The affected third-generation male has children with an unaffected female. They produce four fourth-generation children, including three affected females and an unaffected male. One of the affected second-generation females has children with an affected male. They produce four third-generation children, including two unaffected males and two affected females.
Figure 1. Incidence of protoporphyria in a particular family

Protoporphyria is a genetic disorder characterized by an extreme sensitivity to sunlight. One form of protoporphyria is caused by a mutation in the ALAS2 gene that results in the accumulation of protoporphyrin, an organic compound, in the blood, liver, and skin. The pedigree in Figure 1 shows the incidence of protoporphyria in a particular family.

Which of the following best describes the inheritance pattern illustrated in Figure 1 ?