n.1. in genetics, a gene that appears to have an effect of modifying other hereditary factors but has no significant effect when those other factors are absent. Some investigators believe modifier genes play a role in constitutional medical conditions. Many physical diseases and behaviors are thought to involve modifier genes as well as environmental triggers. 2. in grammar, any word or phrase that qualifies or limits the meaning of another word or phrase (e.g., an adjective or adverb).