Marfan syndrome is an autosomal dominant disorder in humans. It results from mutation of the gene on chromosome 15, which produces the connective tissue protein fibrillin. In its wild-type form, fibrillin gives connective tissues, such as cartilage, elasticity. When mutated, however, fibrillin is rigid and produces a range of phenotypic complications, including excessive growth of the long bones of the leg and arm, sunken chest, dislocation of the lens of the eye, and susceptibility to aortic aneurysm, which can lead to sudden death in some cases.
Different sets of symptoms are seen among various family members, as shown in the pedigree. Each quadrant of the circles and squares represents a different symptom, as the key indicates.
Since all cases of Marfan syndrome are caused by mutation of the fibrillin gene, and all family members with Marfan syndrome carry the same mutant allele, what genetic term best describes the differences shown in the pedigree?