What term is used for the various specific forms of a gene?

The term that refers to the various specific forms of a gene is alleles. Alleles are alternative versions of a gene that arise through mutations and are found at the same locus on homologous chromosomes. Each individual typically carries two alleles for a given gene—one inherited from each parent. These alleles can be dominant or recessive, and the combination of alleles contributed by both parents determines the individual's genotype for that specific trait.

Understanding alleles is crucial for grasping concepts such as genetic variation and inheritance patterns, as they play a significant role in differentiating traits among individuals in a population. For example, in a genetic cross for flower color, different alleles may result in purple or white flowers.

The other terms in the question are related but do not specifically refer to the forms of a gene: phenotypes describe the observable traits resulting from the genotype; chromosomes are structures that organize genetic material; and genotypes refer to the genetic makeup of an individual concerning a trait, which includes the specific alleles present. Thus, alleles are the correct answer as they specifically denote the variations of a single gene.