Mendelian Genetics | from Gregor Johann Mendel (1822–1884), an Austrian monk, experimenting on common garden pea |
| | 1865 – Laws of Particulate Inheritance (dissertation) | describing the principles of transmission of genetic material from one generation to the next |
| | 1900 – rediscovery of Mendel's law | Carl Correns (1864–1933), Hugo de Vries (1948–1935), Erich Tschermak |
| | 1901 – William Bateson (1861–1926), British geneticist | produced the first evidence of inheritance with experiments with chickens |
| | | coincidentally, provided the classical definition of genetics as a field of study, i.e. as a science dealing with heredity and variation seeking to discover laws governing similarities and differences in individuals related by descent |
| | | leading promoter of Mendelian genetics vs Biometricians (biological mathematicians) in the first two decades of the 20th century |
| | | coined technical terms such as homozygote, heterozygote, allelomorph |
| | 1906 – Willhelm Johanssen (1857–1927), Danish botanist | introduced the terms gene, genotype, and phenotype |
Population Genetics | study of Mendelian genetics in populations of plants and animals |
| | basic foundation: Hardy-Weinberg Law | 1908 – Godfrey Harold Hardy (1877–1947), English mathematician |
| | | Willhelm Weinberg (1862–1937), German physician |
| | usually limited to the inheritance of qualitative characters which are influenced by only a small number of (major) genes |
| | study why characteristics become fixed or continue to exhibit variation in natural populations | importance: design of selection strategies to increase frequency of desirable genes or examples: |
| | | Meishan pigs for prolificacy – around 12 offspring |
| | | dwarf gene in poultry |
| | | Booroola gene in sheep for multiple births |
| | | double muscling gene in Pietrain pigs and Belgian blue cattle |
Quantitative Genetics | conceptually the most difficult of the three areas |
| | hypothesis: many genes contribute to expression of traits |
| | effects of individual genes can seldom be seen or measured, e.g. milk yield, growth rate, litter size |
| | complications due to random influence of the environment and other non-genetic factors mask the combined effects of many genes influencing the trait |
| | quantitative genetics is the most important of the three areas because: | response to selection for quantitative traits generally has much more potential monetary value than those for simply-inherited traits |
| | Ronald Aylmer Fisher (1890–1962), British statistician and geneticist, and Sewall Green Wright (1889–1988), American geneticist | reconciled Mendelians and biometricians |
| | Mendelian results: in terms of frequencies of genotypes and phenotypes |
| | biometricians results: in terms of correlations and regressions (before rediscover of Mendel's laws) | e.e. Francis Galton (1822–1911), Karl Pearson (1857–1936) |
| | | Fischer and Wright: demonstrated that Mendelian frequencies were the basis of biometrical correlations |
