Mendel’s laws of inheritance give us basic ideas of inheritance of characters from parents to offsprings. But this inheritance refers to qualitative characters only i.e. traits which are easily classified into distinct phenotypic categories. For example, we find in Mendelian experiments, how many of the plants became tall or dwarf and how many had yellow cotyledons or green cotyledons etc.

The number (of plants) refers to a qualitative trait of the plant but does not let us know “how tall” or “how much yellow” the characters arc. Such phenotypic categories are under the control of one or very few genes with little environmental modifications to obscure the gene effect.

In contrast to this, the variability observed in many crop plants which fail to fit into separate phenotypic classes but forms a spectrum of phenotypes. Technically speaking, the phenotypic classes exemplify “discontinuous variability” and the spectrums of phenotype illustrate continuous variability. Characters such as grain weight yield per acre, milk production, egg production arc quantitative or mctric traits with continuous variability.

The basic difference between qualitative and quantitative traits involves the number of genes contributing to the phenotypic variability and the degree to which the phenotypes arc modified by environmental factors quantitative traits arc generally governed by a large number of genes each contributing to the trait.

ADVERTISEMENTS:

The contribution of each such gene is so small to the phenotype that the individual effects cannot be detected by Mendclian methods. The numbers of genes affecting single traits are together called ‘Polygenes’. Generally speaking, the quantitative characters are influenced more by the environmental factors than by the polygenes.This makes the study complicated much beyond the simple Mendclian genetics.

To make the task easy, geneticists use statistics to arrive at definite conclusions on inheritance pattern in a given environment and to determine the magnitude of the genetic and environmental components to the phenotypic variability.