Chapter 22: Quantitative Genetics and Complex Traits

Quantitative Genetics and Complex Traits begins by distinguishing between discontinuous traits, which follow simple Mendelian patterns, and continuous or quantitative traits—such as height, weight, and crop yield—that are influenced by multiple genes (polygenic inheritance) and environmental factors. The text details the polygene hypothesis using the classic example of wheat kernel color to demonstrate how additive alleles across multiple loci can produce a spectrum of phenotypes. To study these complex traits, the chapter introduces essential statistical tools, including frequency distributions, the mean, variance, and standard deviation, which characterize the central tendency and spread of phenotypic data. It further explains how correlation coefficients measure the strength of associations between variables, while regression analysis allows for the prediction of one variable based on another, such as estimating offspring phenotypes from parental data. A central theme is the concept of heritability, which quantifies the proportion of phenotypic variance attributable to genetic differences versus environmental influences. The chapter rigorously breaks down phenotypic variance into its components: genetic variance (subdivided into additive, dominance, and interactive variance), environmental variance, and genotype-by-environment interactions. It distinguishes between broad-sense heritability, which encompasses all genetic contributions, and narrow-sense heritability, which focuses solely on additive genetic variance and is critical for predicting responses to natural and artificial selection. The breeder's equation is introduced to calculate the response to selection based on the selection differential and narrow-sense heritability. Finally, the chapter explores genetic correlations arising from pleiotropy or linkage and discusses modern methods for identifying Quantitative Trait Loci (QTLs) using molecular markers to map specific genomic regions responsible for continuous variation.