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UM BIOB 272 - Quantitative Genetics
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BIOB 272 1st Edition Lecture 21Outline of Last Lecture Population Genetics (Migration)I. Sickle-cell and Malaria Continueda. Heterozygous AdvantageII. Extreme Changes in Population Sizea. Population Bottlenecksb. Founder EffectsIII. Small Population Effectsa. Inbreedingi. Hapsburg DynastyIV. Population Genetic StructureV. Mathematical Model of Gene FlowVI. Gene Flow EffectsVII. MigrationVIII. Migration and DrifOutline of Current Lecture Quantitative GeneticsI. MigrationII. Migration and Drif ContinuedThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.III. Mechanisms of EvolutionIV. Polygenic Traitsa. Qualitative Traitsb. Quantitative TraitsV. Quantitative GeneticsVI. Nature vs NurtureVII. Heritabilitya. Heritability in the Broad Sense (HBR)b. Heritability in the Narrow Sense (HN or H^2)c. Studies to determine genetic vs environmental influencesi. Twin Studiesii. “Common Garden” Studiesiii. Bell Curve ArgumentCurrent LectureQuantitative GeneticsI. Migration Continued- Gene flow homogenizes populations- Impact of migration depends on migration rate (m)- Gene flow can oppose local selection- But gene flow may also introduce adaptive variationII. Migration and Drif Continued:- Migration and drif both affect ALL loci (selection only some)- Drif and isolation cause populations to diverge and lose genetic variation- Migration homogenizes populations and increases variation within populations- Gene flow interacts with drif to produce population genetic structure visible at neutral marker loci (many applications in conservation biology)- Population genetic structure= drif + migrationIII. Mechanisms of EvolutionMechanism ResultH-W Equilibrium (no evolution)Random mating, no migration, genetic drif of natural selectionAllele frequencies do not changeDrif Imperfect sampling causes some alleles to be underrepresented relative to othersPurple allele is lostNatural Selection Environmental factors are unfavorable for red allelesRed allele becomes less commonMigration (gene flow) Individuals with a new allele enter the populationAllele that enters becomes more commonMutation A red allele becomes an orange alleleNew genetic variant appears in the populationIV. Polygenic Traits- Clicker Question: Consider flower color in elderflower orchid- What kindof trait is this? (A) qualitative or (B) quantitative Answer: Qualitative- has 2 discrete phenotypic classesa. Qualitative Traits (discontinuous characters) : traits that are easily explained by simple model of inheritance- what Mendel studiedb. Quantitative Traits (continuous characters):- Complex genetic basis- Polygenic trait: influenced by many genetic locio Interaction between alleles (epistasis)o Interaction with environment (phenotypic plasticity) V. Quantitative Genetics: Study of the genetic mechanisms of continuous phenotypic traits- Vary continuously- Example: Height- influenced by many genes- Action of many genes usually underlie quantitative traits- Generally influenced by both genetics and environment- Example: Three plants that are cuttings (clones) from a single individual but because of elevation (environmental) differences, they are all different heightsVI. Nature vs Nurture: what proportion of the variation for a trait found in a population has a genetic basis?- Total phenotypic variation for a trait in a population (Vp) arises from two major sources:1. Environmental differences between individuals (VE)2. Genetic differences between individuals (VG)VP= VE + VG- Genetic differences between individuals (VG) can be attributed to 3 sources:1. VA= additive effects (effects of gene substitution)2. VD= dominance effects (effects of allelic interactions at the same locus)3. VI= epistatic effects (effects of interactions between different loci)VP= VE + VGVG= VA + VD + VIVP= VE + VA + VD + VIVII. Heritability: a measure of genetic basis for phenotypic differences between individuals for a trait in a populationa. Heritability in the Broad Sense (HBR): proportion of phenotypic variability that results form genetic differenes between individuals- HB= (VG/VP) = (VA + VD + VI)/ VPb. Heritability in the Narrow Sense (HN or H^2): proportion of the total phenotypic variation that is due to additive genetic differences between individuals- H^2= HN= VA/VP** most traits are in between 0 and 1. 0 meaning no genetic influence and 1 meaning complete genetic influencec. Studies to determine genetic vs environmental influences: main task in determining heritability- Twin Studies: can measure broad sense heritability to estimate the difference in degree of resemblance (r) between monozygotic vs. dizygotic twinso HB= 2(rm-rd)- “Common Garden” Studies: also measure broad sense heritability by comparing clones grown in a “common garden”o HB= VG/ (VG + VE)- Bell Curve Argument:o Educational programs aimed at social (or racial) groups with lower educational performance will not be helpful because intelligence is “genetic”.o This argument is WRONG:– Heritability is a measure of the importance of genetic differences between individuals within the same environment.– Heritability tells us nothing about differences between groups in different environments.– High heritability does not suggest that the phenotype cannot be modified by changing the environment.– Examples: many human diseases, human


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UM BIOB 272 - Quantitative Genetics

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