Quantitative trait variation and evolution

9.1 Quantitative traits

• Components of phenotypic variation.

• Components of genotypic variation (VG).

• Inheritance of additive (VA), dominance (VD), and epistasis (Vj) components of genotypic variation.

• Genotype-by-environment interaction (VGxE).

• Additional sources of phenotypic variation.

In the other chapters of this book, the concept of phenotype employed is somewhat simplistic. This is out of necessity because the emphasis in other chapters is on expectations for genotype and allele frequencies rather than on understanding the causes of variation in phenotype. Phenotypes were assumed to be completely determined by the genotype, and to have two or three discrete classes that correspond exactly to the three genotypes of a single locus with two alleles. (A minor exception is the two-locus model of natural selection where the phenotype is fitness.) While there certainly are examples of phenotypes in natural populations that fit this description, the majority of phenotypes are probably not well characterized by these assumptions. This chapter will expand the concept of phenotype and develop the concepts needed to understand the relationship between various types of genetic variation and phenotypic variation. The chapter will introduce the various components of quantitative trait variation, show how these components can be used to describe inheritance of phenotypes, and also explore the action of natural selection and genetic drift on complex phenotypes. The chapter will wrap up with a section devoted to genetic mapping methods used to identify and characterize individual loci that cause quantitative trait variation.

Think of variable phenotypes such as human height, the number of ears on a corn plant, daily milk production in domestic cows, wood density in a tree species, or the probability of onset of a disease such as diabetes or hypertension. Think next of complex behavioral phenotypes such as sexual preference or propensity to substance addiction in humans, success in male-male contests for mates, or the quality of mates chosen by females. Also think of phenotypes related to Darwinian fitness such as individual size, number of gametes, number of progeny, or number of days an individual survives. While each of these classes of phenotypes seems unrelated, they all share features in common as quantitative traits (also called metric traits). Quantitative traits are sometimes called multifactorial traits because the variation in phenotype among individuals has multiple causes. Quantitative trait variation among individuals is a product of differences in genotype produced by multiple genes as well as differences in environmental conditions experienced by each individual.

The hallmark of quantitative traits is a broad range of variation characterized by a continuous distribution of individual phenotypes in a population (Fig. 9.1). Continuous traits have a scale of measurement that is naturally continuous, such as quantifying height in centimeters or weight in kilograms. Meristic traits exhibit a large number of discrete classes, such as the number of bristles on a fruit fly or the number of leaves on a tree, that forms a distribution of pheno-typic values. Threshold or liability traits are continuously distributed phenotypes with some trait value that defines an upper or lower limit. Trait values above or below the threshold define qualitatively distinct categories such as "normal" and "symptomatic." The production of insulin is one example, where human populations show a continuous distribution of insulin production and individuals are clinically recognized as diabetic when insulin production drops below a threshold level.

In quantitative genetics the words phenotype, trait, and character are all considered synonymous. The term value is used to refer to the phenotype in the same units that it is measured in. Phenotypic value refers to the observed phenotype of an individual, for example observing that an individual fish has a value of 400 mm for the phenotype of body length.

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