Book : Zoology
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Posted by: CHELSEA
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Writer CHELSEA

Quantitive inheritance

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Overview

Charles Darwin while working on his theory of Natural selection, recognised two principal types of variations, namely continuous and discontinuous. The continuous variations show the whole range of variations in a particular character. The discontinuous variations would appear all of a sudden and show no gradation. Mendel in his work depended on sharp or alternate characters comprising discontinuous variations. For example, when tall and dwarf plants were crossed, in F1 only tall plants appeared. Crossing F1 plants, produced only tall and dwarf plants in the F2 generation. Mendel discarded intermediate characters if any, seen. However, by the end of the 19th century Galton, a British geneticist and statistician was interested in the study of continuous variations.

He tried to find an answer for its origin. He called these characters as metrical characters and found them inherited. Thus in the begnning of the 20th century two groups of geneticists emerged. They were the mendelians and the biometricians. While the mendelians considered that all hereditary differences are discontinuous and qualitative, the biometricians belived that hereditary variations are basically continuous and quantitative. These two views remained contradictory. Later, Johansen (1903), through his work on bean seeds proved that both the views of mendelians and biometricians were only partly correct. Yule (1906) suggested that quantitative variations may be controlled by large number of individual genes, with each gene having a small effect. Later on, such genes were called as polygenic systems. The hereditary processes operating through such system was explained through multiple factor hypothesis.


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