Introduction

Let's stop and review quickly where we've come and where we're going. We started our survey of quantitative genetics by pointing out that our objective was to develop a way to describe the patterns of phenotypic resemblance among relatives. The challenge was that we wanted to do this for phenotypic traits that whose expression is influenced both by many genes and by the environment in which those genes are expressed. Beyond the technical, algebraic challenges associated with many genes, we have the problem that we can't directly associate particular genotypes with particular phenotypes. We have to rely on patterns of phenotypic resemblance to tell us something about how genetic variation is transmitted. Surprisingly, we've managed to do that. We now know that it's possible to:

• Estimate the additive effect of an allele.¹

• Partition the phenotypic variance into genotypic and environmental components and to partition the genotypic variance into additive and dominance components.²

• Estimate all of the variance components from a combination of appropriate crossing designs and appropriate statistical analyses.

Now we're ready for the next step: applying all of these ideas to the evolution of a quantitative trait.