Which of the following defines the hereditary units that determine characteristics passed from one generation to the next?

The term that defines the hereditary units responsible for determining characteristics passed from one generation to the next is "genes." Genes are segments of DNA that encode the information necessary to produce proteins, which influence physical traits, behaviors, and other characteristics in an organism.

Genes serve as the fundamental units of heredity and are inherited from parent organisms. They come in different versions known as alleles, which can lead to variations in traits among individuals. While chromosomes are structures that contain many genes, they are not the hereditary units themselves but rather carriers of genes. Loci refer to the specific physical locations of genes on a chromosome but do not define the units of heredity.

In summary, the correct answer is "genes" because they are the actual hereditary units that encapsulate the information required for inheritance, distinguishing them from chromosomes, alleles, and loci.