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Modern Genetic Analysis Chapter 18. Quantitative Genetics
Measures of Central Tendency
The mode.

Most distributions of phenotypes look roughly like those in Figure 18-2: a single most frequent class, the mode, is located near the middle of the distribution, with frequencies decreasing on either side. There are exceptions to this pattern, however. Figure 18-13a shows the very asymmetric distribution of seed weights in the plant Crinum longifolium. Figure 18-13b shows a bimodal (two-mode) distribution of larval survival probabilities for different second-chromosome homozygotes in Drosophila willistoni.

A bimodal distribution may indicate that the population being studied could be better considered a mixture of two populations, each with its own mode. In Figure 18-13b, the left-hand mode probably represents a subpopulation of severe single-locus mutations that are extremely deleterious when homozygous but whose effects are not felt in the heterozygous state in which they usually exist in natural populations. The right-hand mode is part of the distribution of "normal" viability modifiers of small effect.

The mean.

Complete information about the distribution of a phenotype in a population can be given only by specifying the frequency of each measured class, but a great deal of information can be summarized in two statistics. First, we need some measure of the location of the distribution along the axis of measurement (for example, do the individual measurements tend to cluster around 100 cm or 200 cm)? One possibility is to give the measurement of the most common class, the mode. In Figure 18-2b, the mode is 172 cm (for females, the mode would be about 6 cm less). A more common measure of location is the arithmetic average, or the mean. The mean of the measurement (

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