Brachydactyly type D is human autosomal dominant condition in which the thumbs are abnormally short and broad. In most cases, both thumbs are affected, but occasionally just one thumb is involved. The pedigree above shows a family in which brachydactlyly type D is segregating. Filled circles and squares represent females and males who have involvement of both thumbs. Half-filled in symbols represent family members with just one thumb affected?

There is evidence of variable expressivity and incomplete penetrance in this family. Which individual is most likely nonpenetrant for the trait?
◦ III-11
◦ IV-1
◦ IV-5
◦ III-10
◦ II-4