In the case of retinoblastoma, because not all individuals who carry the mutation are affected by the disease, this condition is said to display incomplete or reduced penetrance. Other diseases with incomplete penetrance include Huntington's disease and breast cancer.oreover, both the penetrance and expressivity of different diseases can be partially explained by the action of gene modifiers. Retinoblastoma is a cancer of the retina that primarily affects children and is caused by mutations in the Rb gene.