What are the different forms of a gene called?

The different forms of a gene are referred to as alleles. An allele represents one variant of a gene that may produce different traits or characteristics. For example, a gene like that determining flower color might have a dominant allele for red flowers and a recessive allele for white flowers. Each organism possesses two alleles for each gene (one inherited from each parent), and the interaction of these alleles determines the organism's phenotype, or observable traits.

Other terms listed in the options represent different concepts: phenotypes are the physical expressions of traits resulting from the interaction of alleles; gametes are the reproductive cells that carry alleles to the next generation; and chromatids are the duplicated chromosomes that are joined together before cell division. While all these terms are related to genetics, alleles specifically denote the variations of a gene itself.