Classical genetics is the branch of genetics based solely on visible results of reproductive acts. It is the oldest discipline in the field of genetics, going back to the experiments of Gregor Mendel who made it possible to identify the basic mechanisms of heredity. Subsequently, these mechanisms have been studied and explained at the molecular level.
After the discovery of the genetic code and such tools of cloning as restriction enzymes, the avenues of investigation open to geneticists were greatly broadened. Some classical genetic ideas have been supplanted with the mechanistic understanding brought by molecular discoveries, but many remain intact and in use. Classical genetics is often contrasted with reverse genetics, and aspects of molecular biology are sometimes referred to as molecular genetics.
At the base of classical genetics is the concept of a gene, the hereditary factor tied to a particular simple feature (or character). One can study using the methods of classical genetics a character which contribute differently to multiple genes; such genes are said to be alleles and in most cases there are two: these correspond at the molecular level to the two copies of the gene present in diploid organisms in the two chromosomes. The different ways in which it manifests itself in nature are called phenotypes. The set of genes for one or more characters possessed by an individual is the genotype. An individual whose two alleles for the determination of a character are equal is called homozygous (AA-aa), if different from each other it is called heterozygous (Aa).