Lecture 12 Quantitative Genetics 2024 PDF

HMB265: Human & General Genetics Lecture 12: Quantitative Genetics Professor Belinda Chang Ecology & Evolutionary Biology Dept Cell & Systems Biology Dept Director, Bioinformatics & Genome Biology Chang Lab: Sensory systems Cichlid  Lec...

HMB265: Human & General Genetics Lecture 12: Quantitative Genetics Professor Belinda Chang Ecology & Evolutionary Biology Dept Cell & Systems Biology Dept Director, Bioinformatics & Genome Biology Chang Lab: Sensory systems Cichlid  Lecture Outline Introduction to Quantitative Variation Continuous variation Additive Inheritance P=G+E Reading: Hartwell Genetics textbook, 7th edition, Chapter 2 (emphasis on pages 29-31, 38-39, 48-52)  Phenotypic variation doesnʼt always appear “discrete” (i.e. qualitative), but rather continuous Can genetics explain this kind of variation? Many traits seem quantitative in nature Continuous variation Frequently with a normal distribution Q: Can such traits have a genetic basis? Q: Can simple Mendelian genetics explain this? Many traits influenced by environment Q: Can such traits have a genetic basis? Q: Can simple Mendelian genetics explain this? A trait displaying simple dominance mode of inheritance A trait displaying simple dominance mode of inheritance Dominance is not always complete Summary of dominance relationships Hartwell, Figure 2.1 Incomplete dominance in Antirrhinum A1 A1 A2 A2 A1 A2 A1 A2 A1 A2 A1 A2 A1 A1A1 A1A2 A2A1A2 A2A2 1 A1A1 (red): 2 A1A2 (pink): 1 A2A2 (white) Hartwell, Figure 2.2 But things arenʼt that simple... Additive inheritance Additive inheritance Additive inheritance Additive inheritance Additive inheritance B2 Additive inheritance Additive inheritance Additive inheritance Additive inheritance Additive inheritance Quantitative traits described by a frequency distribution  Genetic origins of a quantitative trait… Two hypotheses: Hypothesis 1: A quantitative trait can be explained on the basis of the segregation of alleles at many loci, where these loci have small, equal, and additive effects. Hypothesis 2: The quantitative trait can be explained on the basis of a few genes with large additive effects. These kinds of traits are said to be polygenic or multifactorial Genetic origins of a quantitative trait… BUT what are we forgetting? Other alterations to Mendelʼs laws Environmental modification Hartwell, Figure 2.23 Consider the environment… Phenotype = Genotype + Environment Genetic origins of a quantitative trait… Genetic origins of a quantitative trait… Genetic origins of a quantitative trait… Occurs with traits that show incomplete dominance Incomplete Dominance The heterozygous phenotype is distinct from either homozygous phenotype - an intermediate phenotype Example: Familial Hypercholesteraemia Incomplete Dominance  Phenotype = Genotype + Environment Quantitative (complex) traits can be explained on the basis of the segregation of alleles at multiple loci with additive effects BUT where individual phenotypic classes are masked by the environment - a quantitative trait locus (QTL) Frequency distributions show the contributions of alleles at a QTL to a complex trait How can we determine if there are genetic effects “buried” within a quantitative trait?