The below mentioned article provides notes on quantitative variation.
The classical experiments in Mendelian inheritance demonstrated transmission of obviously well-marked, distinct pairs of contrasting characters (tall vs. dwarf; yellow vs. green cotyledons, etc.). Moreover, the two alternative expressions of a phenotype were controlled by a single gene which strictly obeyed the rules of dominance to produce visible qualitative differences between phenotypes.
Such traits are said to be qualitative and constitute what are known as discontinuous variations in a population. However, even Mendel’s experiments with pea plants showed small, indistinct, quantitative differences so that all tall plants were not equally tall, nor were all dwarf plants equally short. Instead some degrees of tallness and shortness were noticeable.
Quantitative characters like height, weight and intelligence in man, meat in animals, milk in cattle, all show these variations. These small quantitative variations could not be accounted for by distinct genes because they produced gradations in phenotypes that appeared to be continuous. These small differences were said to result from fluctuations in the environment.
It was at this time that Darwin’s Theory of Evolution which emphasised inheritance of continuous variations was gradually being overthrown by ideas of geneticists like Bateson and De-Vries. The latter proposed that continuous characters were not produced genetically and were not heritable, but only large discontinuous variations were inherited.
At the same time Francis Galton, a cousin of Charles Darwin, and his followers consisting of geneticists and biometricians put forward strong views about the heritable nature of continuous quantitative traits in humans such as height and intelligence. Human beings show all degrees of tallness and shortness between the two extremes.
The biometricians used the terms “metrical” characteristics for such traits which showed small, measurable differences. By analysing the data statistically they could demonstrate the inheritance of gradations in height and intelligence between relatives in human families. But they could not account for the role of genes as separate units.
At the beginning of the 20th Century therefore existed two groups with opposing views. One led by Bateson supporting Mendelian views and emphasizing the heritable nature of qualitative discontinuous traits; the second group led by followers of Galton who contended that heritable variation is basically quantitative and continuous.
The conflict was partially resolved in 1903 when a Danish geneticist W.L. Johannsen analysed a quantitative seed character in Phaseolus vulgaris (bean) and showed that continuous variation was due to both heredity and environment. He established 19 pure lines on the basis of seed weight that ranged from 15 centigrams in the light varieties to 90 centigrams in the heavy varieties.
Johannsen found that the mean seed weight did not differ significantly in the succeeding generations. Although his pure lines were homozygous and true breeding, there were some variations in seed weight within a pure line which he attributed to the environment. It was Johannsen who coined the terms genotype, phenotype and gene in 1903, on the basis of his work on pure lines.
In 1906 Yule suggested that continuous quantitative variation may be due to several genes each with a small effect. However, the controversy was finally resolved by the experiments of the Swedish geneticist Nilsson-Ehle in 1909 who demonstrated that in wheat the quantitative effect was in fact due to many genes. The hypothesis was further confirmed by East in 1910 from his studies on inheritance of corolla length in tobacco.