Qualitative traits are governed by the Mendel’s Laws of inheritance. These are clear cut traits, resulting in two extremes such as the flower colour (purple or white), seed-shape (round or wrinkled) and the seed colour (yellow or green). There is no intermediate phenotype. Variation in qualitative trait is called discontinuous variation.
However, some other important characters in cultivated plants and domestic animals, such as the yield of seeds and fruits as well as the eggs, amount of milk or meat produced by animals, etc., do not fall into two clear-cut classes and all gradations appear between the two extremes, for example, between black and white skin-colour of humans, between red and white colour of wheat grain. These characters are called as quantitative characters. Variation in quantitative trait is called continuous variation.
The problem of the inheritance of quantitative character was taken up by the Swedish Botanist H. Nilsson-Ehle (1908) and American Scientist E. N. East (1910, 1916). These investigators showed that this apparent “blending inheritance” can be explained by supposing that continuously varying characters are due to the combined or cumulative action of several genes, each of which exerts a small effect on the same character. Such genes are called the cumulative genes or additive genes or polygenes.
A cumulative gene is one, which if added to another identical or similar gene, affects the intensity or degree of expression of a quantitative character. In other words, a quantitative character is simultaneously governed by several genes (= polygenes), and the effect or action of such genes is cumulative or additive in nature. This is also known as the multiple-factor hypothesis. Gene-pairs, which act in a cumulative way to result into a quantitative trait, are known as multiple-factors. Since quantitative inheritance is controlled by many genes, it is also known as polygenic inheritance.
Polygenic Inheritance:
“The inheritance which depends on the quantity of alleles of more than one gene is called quantitative inheritance.”
Examples:
(i) Grain colour in wheat
(ii) Ear length in maize
(iii) Corolla length in tobacco
(iv) Skin colour in human beings
(v) Height of human beings
(vi) Weight of human beings
Human skin colour:
Human skin colour is also a quantitative trait. It is controlled by three to six gene pairs. Dark skin is dominant to white skin. The greater the number of pigment specifying genes, the darker the skin is. A child can have darker or lighter skin than his parents.
It is a more complex polygenic trait. The continuous variation of human heights produces a bell-shaped curve in graph. A few people are very tall or very short. But most individuals have average height. This trait is controlled by many pairs of genes at different loci. Even multiple alleles are present at each locus. Shortness is dominant over tallness. More number of alleles for shortness, shorter the height. Similarly, if there are greater numbers of alleles for tallness, the height will be tall.
Effect of environment on continuously varying characters. Environment has a strong influence on height, intelligence and skin colour in human. Constant sunlight darkens skin. Poor nutrition prevents achieving genetically determined height. Healthy and encouraging social environment promotes intelligence.
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