Where does “allele” come from?

Allele comes from German Allel, which was formed from English allelomorph, ultimately from Ancient Greek μορφή meaning "form."

allele (English): One of a number of alternative forms of the same...

Definitions

Step	Language	Word	Meaning
1	German	Allel	allele
2	German	Allelomorph	—
3	English	allelomorph	Synonym of allele
4	Greek	αλληλ-	Alternative form of αλληλο-; reciprocal, mutual;...
5	Ancient Greek	ἄλλος	other, another, different, else; in combination...
6	Proto-Hellenic	áľľos	other, another
7	Proto-Indo-European	h₂élyos	other, another
8	Proto-Indo-European	h₂él-	—

allele — Etymology, Origin & Word History | Etymology Explorer