What could explain why Darren is unaffected but two of his children are affected?

Module 5 b. Donna and Darren have three children [2 male/1 female]. Darren’s father has hypercholesterolemia, which is transmitted in an autosomal dominant pattern. Darren gets his cholesterol levels checked frequently, and they have remained within normal limits; however, two of Donna and Darren’s three children have high plasma levels of total and LDL cholesterol. […]