Introduction to Genetic Analysis 669

Introduction to Genetic Analysis 669 - 44200_20_p643-678...

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Measures of central tendency The mode Most distributions of phenotypes look roughly like those in Figure 20-3: a single mode is lo- cated near the middle of the distribution, with frequen- cies decreasing on either side. There are exceptions to this pattern, however. Figure 20-18a shows the very asymmetrical distribution of seed weights in the plant Crinum longifolium. Figure 20-18b shows a bimodal (two-mode) distribution of larval survival probabili- ties for different second-chromosome homozygotes in Drosophila willistoni. A bimodal distribution may indicate that the popu- lation being studied could be better considered a mix- ture of two populations, each with its own mode. In Fig- ure 20-18b, the left-hand mode probably represents a subpopulation of severe single-locus mutations that are extremely deleterious when homozygous but whose ef- fects 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.
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