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Classical genetics has its origin in the 1850s and 1860s, when the Moravian monk Gregor Mendel attempted to formalise the rules of inheritance governing plant hybridisation. In 1909, the Danish biologist Wilhelm Johannsen proposed that there are two sources of variation: one arising from the action of the environment and the other due to variations in Mendel's hereditary factors – entities Johannsen called ‘genes’. When Mendel's laws were rediscovered in 1900, breeding experiments were wedded to cytological observation of chromosomes in the nucleus of organisms – most notably in the fruit fly – and classical genetics was born. The chromosome theory of inheritance that was formulated as a result of this work stated that genes are located linearly on chromosomes and that chromosomal dynamics underlie the patterns of Mendelian inheritance. Towards mid-century, this work culminated in models of what genes do, and what they are made of. Key Concepts Classical genetics refers to the study of the laws of hereditary transmission in living organisms. Classical genetics has its origin in Gregor Mendel's papers on inheritance in peas, published in 1866. The chromosome theory of inheritance states that genes are located on chromosomes and that chromosomal dynamics underlie the patterns of Mendelian inheritance. The ‘genotype’ is the totality of all the organism's genes, whereas the ‘phenotype’ are the characters displayed, which are products of both the underlying genes and the actions of the environment. A fundamental observation in classical genetics was the ‘one gene, one enzyme’ relationship. It is now known that the relationship between genes and proteins is much more complex. Keywords: breeding; hybridisation; heredity; Gregor Mendel; the Fly Room; one gene one enzyme; transforming principle; double helix; transmission genetics; genotype–phenotype distinction |
