Extra credit questionUses of heritabilityPowerPoint PresentationEfficacy of artificial selection: size of LabradorsBreeder’s QuestionBreeder’s EquationSlide 7Slide 8Exactly the same equation can be used to understand natural selection!Efficacy of natural selection: Darwin’s finchesIf large bills are favored in drought years, what effect will an El Nino year have on the population?R = h2 SHow do you measure the heritability?Another way to measure h2Slide 15Slide 16Most quantitative traits have substantial random environmental varianceHydrangeasSlide 19Slide 20Heritability measured in one population does not tell you anything about differences between populationsSlide 22Slide 23Can these methods be applied to humans?Twin StudiesCorrelations between identical co-twinsCorrelations between co-twins for adult heightHeritability estimates from correlations between co-twinsSlide 29Heritabilty estimates from human twin studies are biasedSlide 31Slide 32ConclusionsMolecular Biology and Quantitative GeneticsQTL MappingSlide 36Slide 37If a marker is far from any gene affecting fruit weight, there will be recombination between the fruit-weight and the markerMany MarkersSlide 40ResultsExtra credit questionThe genes (loci) that influence quantitative traits are known as:_________________________Uses of heritability•The degree to which offspring resemble their parents is determined by the narrow-sense heritability h2•The efficacy of natural and artificial selection is also determined by h2h2 = 1h2 = 0VA/VP = 1VA/VP = 0Efficacy of artificial selection: size of LabradorsBreeder’s QuestionQ: A horse breeder wants to win the Kentucky Derby. If she breeds her mare to a really fast stallion, how likely is it that the colt will be faster than all the other three-year-olds when it runs in the Derby?A: It depends on the heritability of running speed!Breeder’s Equation•R = h2 S•S = Selection differential difference between selected parents and the population as a whole (within a generation)•R = response to selection difference between selected offspring and the unselected population (across generations)Breeder’s EquationR = h2 SA dog breeder chooses his largest dogs to breed together. The average height of the breed is 60 cm (at the shoulder), and the dogs he chooses to breed average 70 cm tall.He knows from previous work that the heritability of height is 0.5.How big can he expect the offspring to be?R = h2 S = 0.5 * 10cm = 5cmBreeder’s EquationR = h2 S = 0.5 * 10 cm = 5 cmIf the response to selection is 5 cm, he can expect his puppies to grow to be60 cm + 5 cm = 65 cm tallExactly the same equation can be used to understand natural selection!Efficacy of natural selection:Darwin’s finchesIf large bills are favored in drought years, what effect will an El Nino year have on the population?h2 = 0.8R = h2 S Before El Nino mean bill depth = 10 cm. Birds that survived the drought had bills that were 2 mm deeper (on average) than the population mean.Q: What happened to the average bill depth in the next generation?A: R = 0.8 * 2mm = 1.6 mm. Bill depth next generation = 10 + 1.6 = 11.6 mm.h2 = 1h2 = 0How do you measure the heritability?Another way to measure h2if R = h2 S, then h2 = R/SA corn breeder chooses to breed from plants that have large cobs. The average cob length in his crop is 15 cm, but he breeds from plants that average 18 cm cobs. Next year, he measures cob length in the offspring of the selected plants and discovers that the mean is 16 cmWhat is the heritability of cob length in this population?h2 = R/S =1 cm/3 cm = 0.33Most quantitative traits have substantial random environmental variance If h2=0.5, then 50% of the phenotypic variance is additive genetic. What’s the other 50%?h2 = VA/VP = VA/(VA+VD+VE)HydrangeasEnvironmental effects on Quantitative Traits: Monozygotic TwinsJones’s Farm Smith’s Farmh2 1.0 1.0milk yield 3 qts/day 2 qts/dayHeritability measured in one population does not tell you anything about differences between populationsEnvironmental variation (VE) is not the same within and between populationsGenetic and Environmental Effects on Quantitative TraitsCan these methods be applied to humans?Environmental variation cannot be controlledTwin Studies•Identical twins share 100% of alleles•Fraternal twins share 50% of alleles•Assume both kinds of twins share VE to the same extentCorrelations between identical co-twinsComplete genetic No genetic determination determination154156158160162164166168170172174176150 155 160 165 170 175Trait in Twin 1Trait in Twin 2r=1.0140145150155160165170175180185190150 155 160 165 170 175Trait in Twin 1Trait in Twin 2r=0.0Correlations between co-twins for adult height150155160165170175180150 155 160 165 170 175Twin 1 HeightTwin 2 Height140145150155160165170175180185190150 155 160 165 170 175Twin 1 HeightTwin 2 HeightMonozygotic (identical) Dizygotic (fraternal)r=0.91r=0.46Heritability estimates from correlations between co-twins•Identical twins share 100% of alleles•Fraternal twins share 50% of allelesH2 ≈ 2* (ri - rf)Height example: ri = 0.91, rf = 0.46H2 = ? H2 = 0.90ri rf Identical Fraternal H2Fingerprint ridges 0.96 0.47 0.98Height 0.90 0.57 0.66IQ score 0.83 0.66 0.34Social maturity 0.97 0.89 0.16Heritabilty estimates from human twin studies are biased1. H2≈2* (ri - rf) overestimates true H2 by 0.5* VD/VP2. H2 includes all genetic effects, not just additive genetic3. GxE interaction increases variability among fraternal twins (it lowers rf) but does not affect ri, so H2 is overestimatedHeritabilty estimates from human twin studies are biased4. Estimates assume identical and fraternal twins share environments to the same extent. Do you think this is true?a) Identical twins share embryonic membrane.b) Identical twins share more similar social environment.Heritabilty estimates from human twin studies are biased5. Studies often based on small sample sizes, and therefore estimates are not very precise (large standard errors)6. Some studies include male-female fraternal twins, whereas identical twins are always the same sex.Conclusions Heritabiltiy estimates derived from human twin studies should be considered very approximate, and probably too high.Molecular Biology and Quantitative GeneticsQTL Mapping•Use DNA-based markers •Marker is anything that differs among strains (RFLPs, microsatellites,