Genetics is the science of genes, heredity, and the variation of organisms, as well as the medical practice of diagnosing, treating, and counseling patients with genetic disorders. Humans began applying knowledge of genetics in prehistory with the domestication and breeding of plants and animals. In modern research, genetics provides important tools in the investigation of the function of a particular gene, e.g. analysis of genetic interactions. Within organisms, genetic information generally is carried in chromosomes, where it is represented in the chemical structure of particular DNA molecules.