Heredity and Variation

Genetics is often described as the science of heredity and variation together, and the two ideas are two sides of the same coin. Heredity is the transmission of characters from parents to their offspring - the reason children resemble their parents. Variation is the difference in characteristics seen among individuals of the same species or population - the reason children are never identical copies of their parents either. Variation itself falls into two contrasting patterns. Discontinuous variation affects traits controlled by one or two major genes with two or more distinct allelic forms - for example plant height in garden pea or style length in Primula - so that individuals fall into sharply separated classes with no intermediate forms, and the environment has essentially no effect on which class an individual belongs to; this is also called qualitative inheritance. Continuous variation, by contrast, arises from the combined action of many genes (polygenes) together with environmental influence, producing a smooth, unbroken gradation between extremes rather than distinct classes - human height and skin colour are classic examples - and is also called quantitative inheritance. Variation matters beyond genetics textbooks: it …