What term describes different versions of the same gene occupying a specific locus on a chromosome?

The correct term for different versions of the same gene occupying a specific locus on a chromosome is "allele." Alleles are variations of a gene that may produce different traits or characteristics. For instance, a gene that determines flower color may have one allele for red flowers and another for white flowers.

Understanding alleles is crucial in genetics because they are responsible for the variations in phenotypic expression among individuals. Each individual inherits two alleles for each gene, one from each parent. This genetic variation is fundamental to the diversity observed within populations.

The other terms do have specific meanings in genetics but do not describe the concept of different versions of a gene. A gene refers to a segment of DNA that encodes a trait. A chromatid is one half of a duplicated chromosome, and a genotype refers to the genetic constitution of an individual, which includes all the alleles present but isn’t restricted to their variations. Thus, "allele" specifically fits the definition of varying versions of the same gene at a particular locus.