Genetics part 1 introduction to advanced genetics
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Mendelian inheritance 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 laws of inheritance were derived by
Gregor Mendel, a nineteenth-century
Austrian monk conducting hybridization experiments in garden peas (
Pisum sativum).[1] Between 1856 and 1863, he cultivated and tested some 5,
000 pea plants. From these experiments, he deduced two generalizations which later became known as Mendel's Principles of
Heredity or Mendelian inheritance. He described these principles in a two-part paper,
Experiments on Plant Hybridization, that he read to the
Natural History Society of
Brno on
February 8 and March 8, 1865, and which was published in 1866.[2]
Mendel's conclusions were largely ignored. Although they were not completely unknown to biologists of the time, they were not seen as generally applicable, even by Mendel himself, who thought they only applied to certain categories of species or traits.
A major block to understanding their significance was the importance attached by
19th-century biologists to the apparent blending of inherited traits in the overall appearance of the progeny, now known to be due to multigene interactions, in contrast to the organ-specific binary characters studied by Mendel.[1] In 1900, however, his work was "re-discovered" by three
European scientists,
Hugo de Vries,
Carl Correns, and
Erich von Tschermak. The exact nature of the "re-discovery" has been somewhat debated: De
Vries published first on the subject, mentioning Mendel in a footnote, while
Correns pointed out Mendel's priority after having read De Vries's paper and realizing that he himself did not have priority. De Vries may not have acknowledged truthfully how much of his knowledge of the laws came from his own work, or came only after reading Mendel's paper.
Later scholars have accused Von Tschermak of not truly understanding the results at all.[1]
Regardless, the "re-discovery" made
Mendelism an important but controversial theory. Its most vigorous promoter in
Europe was
William Bateson, who coined the terms "genetics" and "allele" to describe many of its tenets. The model of heredity was highly contested by other biologists because it implied that heredity was discontinuous, in opposition to the apparently continuous variation observable for many traits. Many biologists also dismissed the theory because they were not sure it would apply to all species. However, later work by biologists and statisticians such as
R. A. Fisher showed that if multiple
Mendelian factors were involved in the expression of an individual trait, they could produce the diverse results observed. Thomas Hunt Morgan and his assistants later integrated the theoretical model of Mendel with the chromosome theory of inheritance, in which the chromosomes of cells were thought to hold the actual hereditary material, and created what is now known as classical genetics, which was extremely successful and cemented Mendel's place in history.
Mendel's findings allowed other scientists to predict the expression of traits on the basis of mathematical probabilities. A large contribution to Mendel's success can be traced to his decision to start his crosses only with plants he demonstrated were true-breeding. He also only measured absolute (binary) characteristics, such as color, shape, and position of the offspring, rather than quantitative characteristics. He expressed his results numerically and subjected them to statistical analysis. His method of data analysis and his large sample size gave credibility to his data. He also had the foresight to follow several successive generations (f2, f3) of pea plants and record their variations.
Finally, he performed "test crosses" (back-crossing descendants of the initial hybridization to the initial true-breeding lines) to reveal the presence and proportion of recessive characters.
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