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Genetics Lectures 15-18.pdf

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McGill University
Biology (Sci)
BIOL 202
Tamara Western

Quantitative GeneticsPolygenic traits influenced by multiple genes usually influenced by environmental conditions This typically leads to continuous variation in the traitRemember Mendelian genes still drive quantitative traitsEnvironmental Effects Genotype NOT EQUAL to the Phenotype You might differ from a heterozygous individual and a homozygous individual if you are AA but not all of your phenotypes are identicalthe can be anywhere in between These may be environmental effects that induce wrikliness in that pea to give a distribution of wrinkles Major environmental effects on how much you eat for exampleMean Vs Variance Meanaverage highest peak in the middle 2Variancehow variable they areVSS N XN i Each individuals value X and subtract the mean value of the populationfrom i2it deviation from the average Square the deviationand divide by NUMBER OF INDIVIDUALS NVtotal phenotypic variance P Vgenetic variance divided into 3 subcategories Go Vadditive genetic variance key for determining direct resemblance Aon average between parents and offspring o Vdominance variance different alleles interact at a given locus One Dbeing dominant over the other o V epistatic variance interaction between different alleles at different I loci with each other Environmental Variance o Vmaternal effects variance if mother had good nutrition while you Mwere in the womb you would have a different phenotype than if you had poor nutrition Ex drinking drugs o V pure environmental variance child born in a colder or warmer Eclimate Or more sunnyless sunny Change their hair colour or skin colour VVVVVVPMEDIAAdditive genetic variance V variance among individuals due to additive effects Aof alleles and genesOne population No interaction between the two alleles A and a you just add up the effects AaTwo populations Pure 1 and Pure 2 genes that determine the differences are additives hybrids should be an intermediate of the two phenotypesV is particularly important because it determines the degree to which Arelatives resemble each otherExample Lake and Inlet Stream Sticklebacks Hybrids in the middle of gill rakers and body depth Dominance variance V variance among individuals due to dominance effects Dbetween allelesAssume that A is completely dominant over a Or crossing 2 Pure breeds as before The hybridAa will look like the dominant phenotypeCan make hybrid backcrosses to the parental form If the differences are additive phenotypes should be proportional to the amount of the genome originating from each group If it deviates from the line the differences are dominant 1 Cross Pure Type 1 with a Pure Type 2results in Pure 1 Pure 2 or F1 hybrid 2 Cross Pure 1 with F1 Hybridresults in Pure 1 F2 hybrid and Backcross to 1 3 Cross Pure 2 with F1 Hybridresults in Pure 2 F2 hybrid and Backcross to 24 Crease a Line Cross Analysis based on theof the genome from Pure 2 Pure 1 0Back 1 25 F1F2 50 Back 2 75 Pure 2 100Backcrosses deviate from the expectation of additivity The backcrosses look more like a lake fish This suggests that alleles for the Lake Type are dominant over the Stream TypeEpistatic variance V variance among individuals due to interactions between Idifferent loci This leads to a variety of possible deviations from additivity and dominance Epistasis the interaction of genes that are not alleles in particular the suppression of the effect of one such gene by anotherFirst look for results to an additive model Linear Cross AnalysisIf not then look to a dominant modelIf not again then there may be a lot of epistatic variance Example with the Soap Berry bug a new golden fruit tree with smaller fruits selected for smaller beaks Long beaks are associated with the native balloon vine fruit Beak length shows huge variationmust be interaction between lociMaternal effects variance V variance among individuals due to the phenotype Mof the motherGraph Mother of one population 1 and fathers of the second 2 And vice versaPure Cross Pop father and mother are same type 11 or 22Hybrid mother from either Pop 1 or Pop 2 and fathers the opposite populationStrong maternal effect irrespective of the genotype of the father Telling you that difference between the two pure types are due to only maternal effects
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