The presence of a defective gene does not necessarily imply a complete expression of every potential manifestation. The variety of ways and levels of severity in which a particular genetic trait manifests among different affected individuals is called expressivity. In neurofibromatosis (NF) 1, for example, an affected child may have only Lisch nodules of the iris and café-au-lait spots. The affected parent may also have extensive punctiform and pedunculated neurofibromas of the skin, plexiform neurofibroma, and optic nerve glioma.
Differences in the age at onset of clinical manifestations are one way that dominant disorders demonstrate expressivity. For example, in NF1, an affected individual may have the following sequence: only café-au-lait spots at birth, Lisch nodules that gradually increase in number and size at about age 5–10 years, punctiform neurofibromas of the skin in early adolescence, subareolar neurofibromas after puberty (females), and visual impairment from the effect of an optic glioma in the late teenaged years. Although all of these features are phenotypic components of the mutant gene, each feature has a characteristic age at onset and a natural history of growth and effect within the umbrella of the total disease. See the section “Genetics of the phakomatoses” for additional discussion of NF1.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.