teratogen (n.)

before vowels terat-, word-forming element meaning "marvel, monster," from combining form of Greek teras (genitive teratos) "marvel, sign, wonder, monster," from PIE *kewr-es-, from root *kwer- "to make, form" (source also of Sanskrit krta- "make, do, perform," Lithuanian keras "charm," Old Church Slavonic čaru "charm").

word-forming element technically meaning "something produced," but mainly, in modern use, "thing that produces or causes," from French -gène (18c.), from Greek -genes "born of, produced by," which is from the same source as genos "birth," genea "race, family," from PIE root *gene- "give birth, beget," with derivatives referring to procreation and familial and tribal groups. First used in late 18th century French chemistry (see oxygen), it probably involves a misunderstanding of -genes, as though it meant "that which produces."