Genetic Modifier By Ivan Suarez Robles 21 Jul, 2011 Genes that affect the phenotype of another gene.Related Articles: HTT Promoter Mutation has a Bi-directional Effect on HD Age of OnsetIdentification of Genetic Modifiers that Alter Clinical Onset of Huntington’s DiseaseDrs. James Gusella and Marcy MacDonaldGenetic Modifiers of Huntington's Disease