What is an allele?

An allele is indeed one of the different forms of a gene. Genes are segments of DNA that influence various traits of an organism, and each gene can exist in multiple versions, which are called alleles. For example, a gene responsible for flower color in a plant may have a red allele and a white allele. The combination of these alleles determines the specific color of the flower.

While the other options relate to genetics, they do not accurately define an allele. A type of chromosome refers to the structure that holds genes, but it doesn't specify the variations of those genes. A visible trait of an organism refers to a phenotype, which is the expression of the alleles rather than the alleles themselves. A combination of traits would describe a genotype or phenotype, but not specifically the variations within a gene that alleles represent. Therefore, identifying an allele as a different form of a gene is the most accurate definition.