Achondroplasia Is a Form of Dwarfism That Is Inherited in Humans

Achondroplasia is a form of dwarfism that is inherited in humans as an autosomal dominant disorder. A survey in a small country showed that, within a two-year period, there were 12 children with normal parents born with this disorder out of a total of 420,000 births. What is the mutation rate in mutations/locus/generation?

A) 1.4 × 10^-5
B) 2.8 × 10^-6
C) 2.5 × 10^-5
D) 2.8 × 10^-5
E) 7.4 × 10^-6