Quantitive inheritance - GENETICS

Charles Darwin while working on his theory of Natural selection, recognised two principal types of variations, namely continuous and discon-tinuous. The continuous variations show the whole range of variations in a particular character. The discontinuous variations would appear all of a sud-den and show no gradation.

Mendel in his work depended on sharp or alternate characters com-prising 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 charac-ters 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 char-acters 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 157 by large number of individual genes, with each gene having a small effect. Later on, such genes were called as polygenic systems. The hereditary pro-cesses operating through such system was explained through multiple factor hypothesis.

Multiple factors

Skin colour in human beings

The inheritance of skin colour in black and white populations in United States of America was worked out and reported by C. B. Davenport in 1913. In USA, marriages between black and white individuals has resulted in a population known as mulattoes. They have intermediate skin colour in the first generation. When the mulattoes marry among themselves, all shades of skin colours are obtained.

If the skin colour is due to genes in two loci A and B, the genotype of black and white persons will be AABB and aabb respectively. The genotypes of mulattoes will beAaBb. However the F2 generation has yielded five or more shades of skin colour. The observed results on number of individuals with differing shades indicate the involvement of four or five gene pairs in the control of skin colour. The effect of these genes may be further modified due to modifying genes.

Studies have also shown that, the control of most characteristics are multifactorial. For example, in human beings several different gene loci are involved in determining characteristics such as body height and body mass.

An understanding of the polygenic influence on a specific genetic trait requires enormous data. The data are to be analysed employing several bio-statistical and biomathematical tools.

From such studies it becomes clear that while at individual level the basic inheritance remains mendelian, at the population level it appears much more complicated.