The previous chapter dealt with the genetic basis of categorical phenotypes such as the presence or absence of a disease. But how do we assess the genetics of quantitative phenotypes that cannot be broken down into distinct categories? Such traits may be affected by a large number of loci acting together, as well as by environmental factors. Important diseaserelated traits such as blood pressure, obesity measures, or cholesterol and triglyceride levels are examples. For such traits, what we need to understand is the effect of the genotypes, and the environment, on the phenotype.

This chapter introduces many important concepts and models for quantitative traits {Crow and Kimura, 1970; Falconer, 1989; Hartl and Clark, 1989; Hedrick, 1985}. The material is rather dense and relentless, but the concepts are important to understand, as they enable us to look further at the genetic causal spectrum, and to decompose it into its constituent parts.

Einstein's trains: assigning phenotypic effects to genotypes

Allelic and genotypic values

Albert Einstein made himself famous by looking out of a train window and speculating as to why he could not tell whether his train was moving north or the train on the next track was moving south.