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| Modern Genetic Analysis Chapter 1. Genetics and the Organism Genetics begins with Variation Figure 1-15. Discontinuous and continuous variation in natural populations. In populations showing discontinuous variation for a particular character, individuals each show one of several discrete alternatives. For example, in the first panel, a population of plants manifests two distinct, common fruit-types, winged and wingless (see Figure 1-14a). Such variation is called polymorphism. Sometimes most of the population is of one kind, with mutation providing only the occasional alternative, as in the case of the vestigial wing type in the fruit fly, Drosophila. Discontinuous variants are often determined by the alleles of a single gene. Continuous variation, on the other hand, does not show such discrete alternatives; a character may be found in phenotypic gradations in a population. There may be no genetic basisall variation may be environmentally causedor genes may be involved, often several or many genes.

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