Mendelian inheritance (or Mendelian genetics or Mendelism or Monogenetic inheritance) is a scientific theory of how hereditary characteristics are passed from parent organisms to their offspring; it underlies much of genetics. This theoretical framework was initially derived from the work of Gregor Johann Mendel published in 1865 and 1866 which was re-discovered in 1900; it was initially very controversial. When Mendel’s theories were integrated with the chromosome theory of inheritance by Thomas Hunt Morgan in 1915, they became the core of classical genetics.
The most influential early theories of heredity were that of Hippocrates and Aristotle. Hippocrates’ theory (possibly based on the teachings of Anaxagoras) was similar to Darwin’s later ideas on pangenesis, involving heredity material that collects from throughout the body. Aristotle suggested instead that the (nonphysical) form-giving principle of an organism was transmitted through semen (which he considered to be a purified form of blood) and the mother’s menstrual blood, which interacted in the womb to direct an organism’s early development. For both Hippocrates and Aristotle—and nearly all Western scholars through to the late 19th century—the inheritance of acquired characters was a supposedly well-established fact that any adequate theory of heredity had to explain. At the same time, individual species were taken to have a fixed essence; such inherited changes were merely superficial.
In the 9th century CE, the Afro-Arab writer Al-Jahiz considered the effects of the environment on the likelihood of an animal to survive.
In 1000 CE, the Arab physician, Abu al-Qasim al-Zahrawi (known as Albucasis in the West) was the first physician to describe clearly the hereditary nature of haemophilia in his Al-Tasrif.
Plant systematics and hybridization
Plant breeders were also developing an array of stable varieties in many important plant species. In the early 19th century, Augustin Sageret established the concept of dominance, recognizing that when some plant varieties are crossed, certain characters (present in one parent) usually appear in the offspring; he also found that some ancestral characters found in neither parent may appear in offspring. However, plant breeders made little attempt to establish a theoretical foundation for their work or to share their knowledge with current work of physiology, although Gartons Agricultural Plant Breeders in England explained their system.
In breeding experiments between 1856 and 1865, Gregor Mendel first traced inheritance patterns of certain traits in pea plants and showed that they obeyed simple statistical rules. Although not all features show these patterns of Mendelian inheritance, his work acted as a proof that application of statistics to inheritance could be highly useful. Since that time many more complex forms of inheritance have been demonstrated.
From his statistical analysis Mendel defined a concept that he described as a character (which in his mind holds also for “determinant of that character”). In only one sentence of his historical paper he used the term “factors” to design the “material creating” the character: ” So far as experience goes, we find it in every case confirmed that constant progeny can only be formed when the egg cells and the fertilizing pollen are of like character, so that both are provided with the material for creating quite similar individuals, as is the case with the normal fertilization of pure species. We must therefore regard it as certain that exactly similar factors must be at work also in the production of the constant forms in the hybrid plants.”(Mendel, 1866).
Mendel’s work was published in 1866 as “Versuche über Pflanzen-Hybriden” (Experiments on Plant Hybridization) in the Verhandlungen des Naturforschenden Vereins zu Brünn (Proceedings of the Natural History Society of Brünn), following two lectures he gave on the work in early 1865.
Mendel’s work was published in a relatively obscure scientific journal, and it was not given any attention in the scientific community. Instead, discussions about modes of heredity were galvanized by Darwin’s theory of evolution by natural selection, in which mechanisms of non-Lamarckian heredity seemed to be required. Darwin’s own theory of heredity, pangenesis, did not meet with any large degree of acceptance. A more mathematical version of pangenesis, one which dropped much of Darwin’s Lamarckian holdovers, was developed as the “biometrical” school of heredity by Darwin’s cousin, Francis Galton. Under Galton and his successor Karl Pearson, the biometrical school attempted to build statistical models for heredity and evolution, with some limited but real success, though the exact methods of heredity were unknown and largely unquestioned.