Both genetic and nongenetic factors cause the congenital skeletal abnormality known as clubfoot, which occurs with a worldwide incidence of about 1 in 1000 births. Gurnett et al. identified a family in which clubfoot occurred as an autosomal dominant trait due to a mutation in the PITXI gene. DNA testing revealed that 11 people in the family carried the PITXI mutation, but only eight of these people had clubfoot. Calculate the penetrance of the PITXI mutation in this family.