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Phenotypic variation: continuous and discontinuous
The measurable physical and biochemical characteristics of an organism, whether observable or not, make up its phenotype. The phenotype results from the interaction of the genotype and the environment. The genotype determines the potential of an organism, whereas the environmental factors to which it is exposed determine to what extent this potential is fulfilled. For example, in humans the potential height of a person is genetically determined, but a person cannot reach this height without an adequate diet. Phenotypic variation (commonly referred to simply as variation) is of two main types: continuous and discontinuous.
In continuous variation, differences are slight and grade into each other. Characteristics such as human height and weight show continuous variation, and are usually determined by a large number of genes (they are polygenic) and/or considerable environmental influence.
In discontinuous variation, the differences are discrete (separate) and clear cut: they do not merge into each other. Discontinuous variations are generally caused by different alleles of one, two, or only a few genes.
Continuous variations are usually quantitative (they can be measured) whereas discontinuous variations are qualitative (they tend to be defined subjectively in descriptive terms). Thus height in humans is a continuous variation given a value in metres, whereas height in sweet peas is a discontinuous variation described as 'tall' or 'dwarf.
Mutations: more variation
Genetic variation arises partly from sexual reproduction by a combination of independent assortment, crossing over, and random fertilisation. However, these processes merely shuffle the existing pack of genes so that new combinations are made. The ultimate source of inherited variations is mutations.
A mutation is a change in the amount or the chemical structure of DNA. If the information contained within the mutated DNA is expressed (that is, transcribed into mRNA and translated into a specific polypeptide chain) it can cause a change in the characteristics of an individual cell or an organism. Mutations in the gametes of multicellular organisms can be inherited by offspring. Mutations of the body cells of multicellular organisms (somatic mutations) are confined to the body cells derived from the mutated cell; they are not inherited.
Mutations can happen spontaneously as a result of errors in DNA replication or errors during cell division, or they can be induced by various environmental factors (such as certain chemicals, X-rays, and viral infection). Factors that induce mutations are called mutagens.