Understanding the genetic basis of any characteristic begins with a description of the numbers and kinds of phenotypes present in a group of individuals. Phenotypic variation in a group, such as the progeny of a cross, can be conveniently represented by a frequency distribution, which is a graph of the frequencies (numbers or proportions) of the different phenotypes (I Figure 22.6). In a typical frequency distribution, the phenotypic classes are plotted on the horizontal (x) axis and the numbers (or proportions) of individuals in each class on the vertical (y) axis. Unlike qualitative characteristics (IFigure 22.6a), quantitative characteristics often exhibit many phenotypes, so a frequency distribution is a concise method of summarizing them all (I Figure 22.6b).

Connecting the points of a frequency distribution with a line creates a curve that is characteristic of the distribution (iFigure 22.7). Many quantitative characteristics exhibit a

(a) Qualitative

(discontinuous) characteristic

(b) Quantitative (continuous) characteristic


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