UIUC IB 201 - Genetics of Complex Traits Quantitative Genetics

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Genetics of Complex Traits: Quantitative GeneticsGenetic VariationPowerPoint PresentationSlide 4Slide 5Two additive genes: discrete phenotypic distributionColor of wheat kernels: three additive genesFrequency Distribution of Height of the BandProperties of distributionsSlide 10Types of VariancePhenotypic VarianceSlide 13Additive and Dominance Effects (No Environmental Effects)Slide 15Slide 16HeritabilityUses of heritabilitySlide 19Slide 20Efficacy of artificial selection: size of LabradorsBreeder’s QuestionBreeder’s EquationSlide 24Slide 25Exactly 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 SGenetics of Complex Traits:Quantitative GeneticsGenetic VariationContinuous Variation (height)Discrete Variation (presence/absence of tail)PolygenicEnvironmental influencesHave continuous (not discrete) distributionsCan be measured on a quantitative scaleHeightWeightAthletic abilityRisk of heart diseaseRisk of diabetesRisk of cancer Quantitative Genetics121012GenotypeFrequencyRR Rr rrDiscrete distributionPartial dominanceIntermediate dominance= “additive” gene actionF2: AB Ab aB abAB AABB AABb AaBB AaBbAb AABb AAbb AaBb AabbaB AaBB AaBb aaBB aaBbab AaBb Aabb aaBb aabbRedLt RedPinkLt PinkLt RedPinkLt RedPinkPinkLt PinkLt RedPinkPinkLt PinkLt PinkwhiteP:AABB redx aabb whiteF1: AaBb x AaBb All Pink1 Red: 4 Lt Red: 6 Pink: 4 Lt Pink: 1 whiteTwo additive genes: discrete phenotypic distribution146410123456Red Lt Red Pink Light Pink WhiteColor of wheat kernels: three additive genesaabbcc AABBCCAaBbCCFrequency Distribution of Height of the Bandmean=68 inchesMean = = 68 inches€ x =xii=1n∑n Variance = = 9.5 in2€ σ2= Va r =xi− x( )2i=1n∑n −1( ) Properties of distributionsn =160Types of VariancePhenotypic variance: total variance of the population, includes variation from genes and from the environmentGenetic variance: the variance that is due to variation among individuals in the alleles that they have, excludes environmentally-caused variationPhenotypic Variance Var = 9.5 in2Phenotypic variance = Genetic variance + Environ. variance VP = VG + VEMean = 68 inGenetic variance = Additive variance + Dominance Variance VG = VA + VDVP = VA + VD + VEPhenotypic variance = Genetic variance + Environ. variance VP = VG + VEDominance effectsAdditive effectsAAAaaaBBBbbb+2+2+0+2+1+0P:AABBxaabb20cm16 cmF1:AaBb 19cmF2 Genotypes:AABBAABbAAbbAaBBAaBbAabbaaBBaaBbaabbGenotypicEffects+4+3+2+4+3+2+2+1+0Phenotype(cm)201918201918181716F2 proportions:1/162/161/162/164/162/161/162/161/16Additive and Dominance Effects (No Environmental Effects)Dominance effectsAdditive effectsAAAaaaBBBbbb+2+2+0+2+1+0F1:Aa BbxAa Bb19 cm19 cmF2 Genotypes:AABBAABbAAbbAaBBAaBbAabbaaBBaaBbaabbGenotypicEffects+4+3+2+4+3+2+2+1+0Phenotype(cm)201918201918181716F2 proportions:1/162/161/162/164/162/161/162/161/16VP = VA + VD + VEVar = 1.333 cm21.333 = 1.0 + 0.333 + 0Mean = 18.5 cm00.10.20.30.416 17 18 19 20Length in inchesFrequency in populationVP = 1.333 cm2VA = 1.0 VD = 0.333 VE = 0HeritabilityBroad-sense heritability H2 = VG/VPNarrow-sense heritability h2 = VA/VPVP = VA + VD + VE1.333 = 1.0 + 0.333 + 0 = 1.0 = 0.75Uses 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 speedBreeder’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 Birds that survive the drought have bills that are 2 mm deeper (on average) than the population mean.Q: What will happen to the average bill depth in the next


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UIUC IB 201 - Genetics of Complex Traits Quantitative Genetics

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