Understand the terminology: In genetics, a gene is a segment of DNA that codes for a specific trait.
Recognize that a gene can have different versions or forms, which are called alleles.
Differentiate between the terms: a chromosome is a structure that contains many genes; phenotype is the observable trait; genotype is the genetic makeup.
Identify that the question asks for the term describing one form of a gene, which corresponds to 'allele'.
Conclude that the correct answer is 'allele' because it specifically refers to one variant of a gene.