It is the oldest discipline in the field of genetics, going back to the experiments on Mendelian inheritance by Gregor Mendel who made it possible to identify the basic mechanisms of heredity.
The observation that some genes do not segregate independently at meiosis broke the laws of Mendelian inheritance and provided science with a way to map characteristics to a location on the chromosomes.
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.
At the base of classical genetics is the concept of a gene, the hereditary factor tied to a particular simple feature (or character).
In the sense of classical genetics, variation is known as the lack of resemblance in related individuals and can be categorized as discontinuous or continuous.
Gregor Mendel started his experimentation and study of inheritance with phenotypes of garden peas and continued the experiments with plants.
About 150 years ago, Gregor Mendel published his first experiments with the test crossing of Pisum peas.
The results of this experiment allowed him to see which of these two traits was dominant and which was recessive based upon the number of offspring with each phenotype.