the neurofibromin gene NF1 on chromosome 17. A loss of function mutation in the tumor suppressor gene can cause tumors on the nerves called neurofibromas. These appear as small bumps under the skin. It is stipulated that the phenotypic variation is a result of unlinked genetic modifiers. See also Anticipation Pleiotropy Mendelian inheritance Genetic heterogeneity Haploinsufficiency References Further reading Population genetics