Extra credit question The 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 h 2 h2 1 h2 0 VA VP 1 VA VP 0 Efficacy of artificial selection size of Labradors Breeder s Question Q 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 Equation R h2 S A 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 5cm Breeder s Equation R h2 S 0 5 10 cm 5 cm If the response to selection is 5 cm he can expect his puppies to grow to be 60 cm 5 cm 65 cm tall Exactly the same equation can be used to understand natural selection Efficacy of natural selection Darwin s finches h 0 8 2 If large bills are favored in drought years what effect will an El Nino year have on the population R 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 How do you measure the heritability h2 1 h2 0 Another way to measure h2 if R h2 S then h2 R S A 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 cm What is the heritability of cob length in this population h2 R S 1 cm 3 cm 0 33 Most quantitative traits have substantial random environmental variance h2 VA VP VA VA VD VE If h2 0 5 then 50 of the phenotypic variance is additive genetic What s the other 50 Hydrangeas Environmental effects on Quantitative Traits Monozygotic Twins Heritability measured in one population does not tell you anything about differences between populations Environmental variation VE is not the same within and between populations Jones s Farm h2 1 0 milk yield 3 qts day Smith s Farm 1 0 2 qts day Genetic and Environmental Effects on Quantitative Traits Can these methods be applied to humans Environmental variation cannot be controlled Twin Studies Identical twins share 100 of alleles Fraternal twins share 50 of alleles Assume both kinds of twins share VE to the same extent Correlations between identical co twins Complete genetic determination No genetic determination 176 174 172 170 190 r 1 0 185 180 168 175 166 170 164 165 162 160 160 155 Trait in Twin 2 150 Trait in Twin 2 158 145 156 154 150 r 0 0 155 160 165 Trait in Twin 1 170 175 140 150 155 160 165 Trait in Twin 1 170 175 Correlations between co twins for adult height Monozygotic identical Dizygotic fraternal 180 175 190 r 0 91 185 180 r 0 46 175 170 170 165 165 160 160 155 Twin 2 Height Twin 2 Height 150 155 145 150 150 155 160 165 Twin 1 Height 170 175 140 150 155 160 165 Twin 1 Height 170 175 Heritability estimates from correlations between co twins Identical twins share 100 of alleles Fraternal twins share 50 of alleles H2 2 ri rf Height example ri 0 91 rf 0 46 H2 H2 0 90 ri rf Identical Fraternal H2 Fingerprint ridges 0 96 0 47 0 98 Height 0 90 0 57 0 66 IQ score 0 83 0 66 0 34 Social maturity 0 97 0 89 0 16 Heritabilty estimates from human twin studies are biased 1 H2 2 ri rf overestimates true H2 by 0 5 VD VP 2 H2 includes all genetic effects not just additive genetic 3 GxE interaction increases variability among fraternal twins it lowers rf but does not affect ri so H2 is overestimated Heritabilty estimates from human twin studies are biased 4 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 biased 5 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 Genetics QTL Mapping Use DNA based markers Marker is anything that differs among strains RFLPs microsatellites single base pair differnces Markers close to genes will tend to be inherited along with genes recombination rare Small m s Large m s M S m s F1 M S M S Markers that are close to the QTL are inherited with it recombination is rare m s F1 M S Markers that are close to the QTL are inherited with it recombination is rare F2 m s m s Small m s M S Intermediate M S M S Large If a marker is far from any gene affecting fruit weight there will be recombination between the fruitweight and the marker A marker far from a gene will not show a statistical association with the phenotype Many Markers Results 28 QTLs responsible for fruit weight variation in tomato One gene has been cloned and transferred between plants When small allele is transferred to normally large fruited variety the fruit weight is reduced by 30 Proof that the gene is really a QTL
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