Lecture 13 QTLs 2024 PDF

HMB265: Human & General Genetics Lecture 13: Quantitative Trait Loci Prof. Belinda Chang  Lecture Outline Quantitative Trait Analysis Heritability Broad Sense Heritability Narrow Sense Heritability Reading: Goldberg et al 2024, 8th ed. Genetics textbook,...

HMB265: Human & General Genetics Lecture 13: Quantitative Trait Loci Prof. Belinda Chang  Lecture Outline Quantitative Trait Analysis Heritability Broad Sense Heritability Narrow Sense Heritability Reading: Goldberg et al 2024, 8th ed. Genetics textbook, Chapter 25 (emphasis on pages 764-771)  How can we determine if there are genetic effects “buried” within a quantitative trait? How much of this variation is attributable to genetic variation? How can we determine if there are genetic effects “buried” within a quantitative trait? How much of the phenotypic variation is attributable to genetic variation versus environmental variation?  How can we determine if there are genetic effects “buried” within a quantitative trait? The extent of phenotypic variation that is attributable to genetic variation is known as the BROAD SENSE HERITABILITY of the trait If all the phenotypic variation is attributable to genotypic variation, then H2 = 1 (the maximum it can be) If all the phenotypic variation is attributable to environmental effects, then H2 = 0 (the minimum)  What does H2 tell us? If a lot of the phenotypic variation is attributable to genotypic variation, then H2 is close to 1 High values suggest that genotype is important in determining whether the quantitative trait arises due to segregation of particular alleles BUT H2 does not predict how progeny will perform on the basis of the phenotype of their parents H2 for some traits in humans  What H2 does and does not tell us What H2 tells us If H2 is high, the phenotype of an individual is likely to be attributable to its genotype IN THAT FAMILY What H2 does NOT tell us What phenotype an individual will have on the basis of its parentsʼ phenotypes Even if H2 is high, an individualʼs precise phenotype cannot be predicted on the basis of its parentsʼ phenotypes What is going on in other families - H2 is family specific What H2 tells us Look like they do because of their genes What H2 does NOT tell us What their offspring will look like Why is this the case?  Why H2 is not predictive VP = VG + VE VG = VA + VD + VI VA variation due to additive effects VD variation due to dominance effects VI variation due to epistatic effects  Why H2 is not predictive VD = variation due to dominance effects  Why H2 is not predictive VD = variation due to dominance effects Why H2 is not predictive VI = variation due to epistatic effects Why H2 is not predictive Vi = variation due to epistatic effects  Why H2 is not predictive VP = VG + VE VG = VA + VD + VI VA variation due to additive effects VD variation due to dominance effects VI variation due to epistatic effects If the trait is due to additive effects, it is predictive… What we need is heritability based on additive variation The extent of phenotypic variation that is attributable to additive genetic variation is known as NARROW SENSE HERITABILITY If all the phenotypic variation is attributable to additive variation, then h2 = 1 (the maximum it can be) If all the phenotypic variation is attributable to other genetic & environmental effects, then h2 approaches 0 What h2 does and does not tell us What h2 tells us If h2 is high, the phenotype of an individual is predictable based on the phenotype of its parent IN THAT FAMILY Specific additive-effect genes are involved What h2 does NOT tell us What is happening in other families What the genes are The heights of individuals and their same-sex parent are correlated h2 for some traits in different species