Q.Differentiate continuous variation from discontinuous variation.

Step 1. Discontinuous variation occurs in traits controlled by one or two major genes, which may have two or more allelic forms; individuals produced by this kind of variation fall into sharply separated classes, with no intermediate forms and no overlap between the phenotype classes - style length in Primula and plant height in garden pea are the examples cited.

Step 2. This kind of variation is also called qualitative inheritance, because its expression is essentially unaffected by environmental conditions - the phenotype class an individual falls into is set almost entirely by its genotype.

Step 3. Continuous variation, by contrast, arises from the combined effects of many genes acting together (polygenes) plus environmental factors, so that most individuals in a population show a complete, unbroken gradation from one extreme to the other, with no distinct classes at all - human height and skin colour are the classic examples. …

Contrast the number of controlling genes, the presence/absence of distinct classes, and the role …

• Forgetting that discontinuous variation is essentially unaffected by environment, while continuous variation is shaped by it. …