Meaning

noun (genetics) either of a pair (or series) of alternative forms of a gene that can occupy the same locus on a particular chromosome and that control the same character

originFrom allelo- (“other”) + -morph (“shape”), from Ancient Greek ἀλλήλων (allḗlōn, “one another, each other, mutually, reciprocally”) from ἄλλος (állos, “other”). Coined by English biologists William Bateson and Edith Rebecca Saunders in 1902, in a paper titled "The facts of heredity in the light of Mendel’s discovery".

Etymology from Wiktionary, CC BY-SA 3.0