Penetrance

Lore & Background

Penetrance can be complete, where 100% of individuals with a genotype express the trait, as seen in neurofibromatosis type 1. Reduced penetrance occurs when less than 100% express the trait, as with BRCA1 mutations, where about 65% of women develop breast cancer by age 70. Non-penetrance refers to individuals carrying a mutation without any signs or symptoms, allowing the mutation to be passed to future generations.

Reader's Guide

Penetrance is a key concept in medical genetics for understanding disease risk and inheritance patterns. It is influenced by factors such as age, sex, environment, epigenetic modifications, and modifier genes. Age-dependent penetrance is seen in C9orf72 repeat expansions, which are non-penetrant until age 35 and nearly completely penetrant by age 80. Gender-related penetrance is exemplified by BRCA2 mutations, with much higher breast cancer penetrance in women than men. Estimating penetrance is challenging due to ascertainment bias in family-based studies and healthy-participant bias in population studies. Phenocopies—environmentally caused illnesses that mimic genetic diseases—further complicate penetrance determination. Understanding penetrance helps in genetic counseling and risk assessment, though estimates must account for multiple influencing factors.

Did You Know?

• Neurofibromatosis type 1 shows complete penetrance, meaning everyone with the disease-causing variant develops some symptoms.
• BRCA1 mutation has reduced penetrance: about 65% of women carriers develop breast cancer by age 70.
• A C9orf72 repeat expansion is non-penetrant until age 35, 50% penetrant by age 60, and almost completely penetrant by age 80.
• A study of monozygotic twins found that increased methylation of the BRCA1 gene in one twin was linked to cancer, while the other twin remained healthy.