Brachydactyly type d is a 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 accompanying pedigree shows a family in which brachydactyly type d is segregating. filled circles and squares represent females and males who have involvement of both thumbs. half-filled symbols represent family members with just one thumb affected.

A. The term variable expressivity refers to the differences that can be observed in the phenotype of individuals with the same genotype. This phenomenon can explain the range of signs and symptoms seen in people with the same genotype. The pedigree provided shows that the family has individuals with one affected thumb and individuals with two affected thumbs. These individuals have the same genetic condition but their symptoms are different and this means that there is evidence for variable expressivity of this specific genetic trait in this family.

B. There are cases where an individual carries a specific gene but does not express the phenotype connected to this gene. This phenomenon is called incomplete penetrance. The pedigree shows that unaffected parents (III-5 and III-10), have affected children who express the trait. This means that these parents carry the autosomal allele for this trait and passed on to their offsprings.