Twо brоthers аre аffected with neurоfibromаtosis type 1. They inherited a mutation in the NF1 gene from their mildly affected father. One brother has multiple cutaneous neurofibromas, mental retardation and an optic glioma (brain tumor). The other brother has only cafe au lait spots and axillary freckling. The difference in phenotypes between the two brothers is an example of which of the following principles?