Douglas Harper's Etymology Dictionary
pheno-
before vowels phen-, word-forming element in science meaning "pertaining to or derived from benzene," from phene.
Wiktionary
pheno-
pre. 1 (context biology English) Prefix indicating an observable characteristic. Example: phenotype. 2 (context organic chemistry English) Used in IUPAC names to indicate the presence of a phenol group. Example: phenobarbital.