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The term allele, from Greek allelos, refers to each of the different possible forms or versions of the same gene, relating to the same character.
A chromosome, from the Greek khroma: color and soma: body, is a filament contained in the nucleus of the cells.
Chromatide is the name given to the two "arms" of a chromosome at the beginning of cell division. The two chromatids of a chromosome are connected by the centromere.
