Final answer:
The genetic principle that explains the varying degrees of symptoms in people with neurofibromatosis is expressivity, which describes how broadly the genotype is expressed phenotypically.
Step-by-step explanation:
The principle that explains the varying degree of symptoms in people with neurofibromatosis is expressivity. Expressivity refers to the range of symptoms or traits that can occur in different individuals possessing the same genotype. It is the degree to which a genotype is expressed in an individual's phenotype. In the case of neurofibromatosis, which is an autosomal dominant disorder, individuals with the same Nn genotype might exhibit a wide range of physical symptoms due to variable expressivity. This is different from penetrance, which refers to the likelihood that a person with a certain genotype will express any traits associated with a genetic condition, and from dominance or recessiveness, which refers to how alleles interact to create phenotypes in cases of heterozygosity.