Final answer:
An organism's traits are determined by the specific combination of inherited genes, which have different versions called alleles. These genes are located on chromosomes and their expression leads to an organism's phenotype.
Step-by-step explanation:
The inherited feature that, in specific combinations, determines an organism's traits is genes. Genes are located on chromosomes, which are matched pairs of DNA carrying the blueprint of life. They are the functional units that determine specific characteristics by coding for specific proteins. An organism's traits are the variations of these characteristics. The gene for a characteristic may have different versions, known as alleles. Alleles account for the variation observed in the traits of organisms. For instance, in pea plants, different alleles determine the color of the flowers.
The traits that an organism expresses are considered its phenotype, while the genetic makeup is referred to as the genotype. The understanding of inheritance and trait determination was greatly advanced by the work of Gregor Mendel, who used pea plants to study the way traits are passed on from one generation to the next.