遗传分析-第二十五章Quantitative Genetics.doc

Chapter 25 Quantitative GeneticsKey Concepts In natural populations, variation in most characters takes the form of a continuous phenotypic range rather than discrete phenotypic classes. In other words, the variation is quantitative, not qualitative. Mendelian genetic analysis is extremely difficult to apply to such continuous phenotypic distributions, so statistical techniques are employed instead. A major task of quantitative genetics is to determine the ways in which genes interact with the environment to contribute to the formation of a given quantitative trait distribution. The genetic variation underlying a continuous character distribution can be the result of segregation at a single genetic locus or at numerous interacting loci that produce cumulative effects on the phenotype. The estimated ratio of genetic to environmental variation is not a measure of the relative contribution of genes and environment to phenotype. Estimates of genetic and environmental variance are specific to the single population and the particular set of environments in which the estimates are made. Introduction Ultimately, the goal of genetics is the analysis of the genotype of organisms. But the genotype can be identified — and therefore studied—only through its phenotypic effect. We recognize two genotypes as different from each other because the phenotypes of their carriers are different. Basic genetic experiments, then, depend on the existence of a simple relation between genotype and phenotype. That is why studies of DNA sequences are so important, because we can read off the genotype directly from this most basic of all phenotypes. In general, we hope to find a uniquely distinguishable phenotype for each genotype and only a simple genotype for each phenotype. At worst, when one allele is completely dominant, it may be necessary to perform a simple genetic cross to distinguish the heterozygote from the homozygote. Where possible, geneticists avoid studying genes that have only p