Introduction to quantitative genetics - Holsinger Lab

1 Introduction to quantitative geneticsSo far in this course we have dealt entirely either with the evolution of characters that arecontrolled by simple Mendelian inheritance at a single locus or with the evolution of molecularsequences. Even last week when we were dealing with population genomic data, data fromhundreds or thousands of loci, we were treating the variation at each locus separately andcombining results across loci. I have some old notes on gametic disequilibrium and howallele frequencies change at two loci simultaneously, but they re on the course website inthe Old notes, no longer updated section and we didn t discuss every examplewe ve considered we ve imagined that we could understand something about evolution byexamining the evolution of a single gene.

2 That s the domain of classical population the next few weeks we re going to be exploring a field that s older than classicalpopulation genetics , although the approach we ll be taking to it involves the use of populationgenetic you know a little about the history of evolutionary biology, you mayknow that after the rediscovery of Mendel s work in 1900 there was a heated debate betweenthe biometricians ( , Galton and Pearson) and the Mendelians ( , de Vries, Correns,Bateson, and Morgan).Biometricians asserted that the really important variation in evolution didn t followMendelian rules.

3 Height, weight, skin color, and similar traits seemed to vary continuously, show blending inheritance, and show variable responses to the variation in suchquantitative traitsseemed to be more obviously related to organismaladaptation than the trivial traits that Mendelians studied, it seemed obvious to the bio-metricians that Mendelian geneticists were studying a phenomenon that wasn t will spend some time talking about gametic disequilibrium when we talk about association mappingin a couple of fact, it involves the use of the single-locus population genetic machinery we ve been using all 2001-2019 Kent E.

4 HolsingerFigure 1:Hypothetical illustration of reaction norms for two genotypes acrossa1-dimensionalenvironmentalgradien t(fromWikipedia,PublicDomain, , accessed 9 April 2017).Mendelians dismissed the biometricians, at least in part, because they seemed not torecognize the distinction between genotype and phenotype. It seemed to at least someMendelians that traits whose expression was influenced by the environment were, by defi-nition, not inherited. Moreover, the evidence that Mendelian principles accounted for theinheritance of many discrete traits was s [3] experiments onDaphniahelped to show that traits whose expression isenvironmentally influenced may also be inherited.

5 He introduced the idea of anorm of reac-tionto describe the observation that the same genotype may produce different phenotypes indifferent environments (Figure 1). When you fertilize a plant, for example, it will grow largerand more robust than when you don t. The phenotype an organism expresses is, therefore,a product ofbothits genotype and its s [2] experiments on inheritance of kernel color in wheat showed how contin-uous variation and Mendelian inheritance could be reconciled (Figure 2). He demonstratedthat what appeared to be continuous variation in color from red to white with blendinginheritance could be understood as the result of three separate genes influencing kernel colorthat were inherited separately from one another.

6 It was the first example of what s come tobe known aspolygenic inheritance. Fisher [1], in a paper that grew out of his undergraduateHonors thesis at Cambridge University, set forth the mathematical theory that describeshow it all works. That s the theory ofquantitative genetics , and it s what we re going tospend the next several weeks 2: Results from one of Nilsson-Ehle s crosses illustrating polygenic inhertance of ker-nel color in wheat ( , accessed 9 April2017).3An overview of where we re headedWoltereck s ideas force us to realize that when we see a phenotypic difference between twoindividuals in a population there are three possible explanations for that difference:1.

7 The individuals have different The individuals developed in different The individuals have different genotypesandthey developed in different leads us naturally to think that phenotypic variation consists of two separable compo-nents, namely genotypic and environmental that into an equationVar(P) = Var(G) + Var(E),where Var(P) is thephenotypic variance, Var(G) is thegenetic variance, and Var(E) isthe environmental we ll see in just a moment, we can also partition thegenetic variance into components, theadditive genetic variance, Var(A), and thedominancevariance, Var(D).

8 5 There s a surprisingly subtle and important insight buried in that very simple equation:Because the expression of a quantitative trait is a result both of genes involved in thattrait s expression and the environment in which it is expressed, it doesn t make sense to sayof a particular individual s phenotype that genes are more important than environment indetermining it. You wouldn t have a phenotype without both. At most what we can say isthat when we look at a particular population of organisms some fraction of the phenotypicvariation they exhibit is due to differences in the genes they carry and that some fractionis due to differences in the environment they have we have two individualswith different phenotypes, , Ralph is tall and Harry is short.

9 We can t even say whetherthe difference between Ralph and Harry is because of differences in their genes or differencesin their developmental ll soon see that separating genotypic and environmental components is far from trivial. I m alsoputting aside, for the moment, that genotypes may differ in their response to the environment, even thoughthat s what I illustrated in discussing norms of speaking we should also include a term for the interaction between genotype and environment,but we ll ignore that for the time being.

10 I illustrated the interaction between genotype and environment indiscussing norms of could even partition it further into additive by additive, additive by dominance, and dominance bydominance epistatic variance, but let s not go I put it this way, I hope it s obvious that I m neglecting genotype-environment interactions, andthat I m oversimplifying a important implication of this insight is that much of the nature vs. nurture debateconcerning human intelligence or human personality characteristics is misguided. The intel-ligence and personality that you have is a product ofboththe genes you happened to inheritand the environment that you happened to experience.