In This Chapter
^ Teasing apart phenotypic variation into genetic and environmental components ^ Making sense of broad- and narrow-sense heritability ^ Measuring the response to selection on quantitative traits any of the traits scientists are interested in aren't the result of a single gene, but of multiple genes. As discussed in previous chapters, some traits are controlled by one gene (or by a known few genes). In contrast, quantitative (or multigenetic) traits are determined by multiple (more than two) genes.
When multiple genes and environmental factors, which can have a big impact on how traits are expressed, both play significant roles in the phenotype, how much of the phenotype depends on the genes (and therefore is heritable), and how much depends on the environment (and is not heritable)?
If you expect that the next question is "Why in the world would anyone care?" remember that in evolution, only heritable variation is important. Many of the traits scientists, agriculturalists, and medical professionals care about turn out to be quantitative traits. The field of quantitative genetics deals with determining what proportion of the variation is due to genetic factors when multiple genes and the environment play a role.
Geneticists in this field use all sorts of mathematical formulas and statistical techniques to figure out all this information, none of which you need to know. So this chapter skips the math and focuses on the key principles instead. Here, I paint with a broad brush but give you the foundation you need to understand this topic — just in case you run across it in the science section of the newspaper.
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