Quantitative Genetics - Western Oregon Universityguralnl/311Quantitative Genetics.pdf ·...
Transcript of Quantitative Genetics - Western Oregon Universityguralnl/311Quantitative Genetics.pdf ·...
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Quantitative Genetics• Polygenic inheritanceContinuous variation Additive alleles Calculating the number of
genes
• HeritabilityStatistical tools: Mean,
variance Broad sense heritability Narrow sense heritability Correlation: Twin Studies
and concordance
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Polygenic inheritance
• 2 or more genes• Show continuous
variation vsdiscontinuous
• Additive component• Distinct phenotypic
classes• Quantitative traits:
size, weight, height,IQ
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Polygenic inheritance
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Continuous variation
• Kolreuter’s cross
• Dwarf x tall tobacco
• F1 intermediate
• F2 intermediate, normal distribution
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Multiple gene hypothesis
• East’s cross of Nicotiana with different corolla length
• Indicates Mendeliansegregation of different phenotypic classes
• Took subsets of F2and crossed.
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Multiple factor hypothesis
• Characters quantified• Two or more genes• Additive alleles• Contribute a constant
amount• Non-additive add
nothing• All alleles add equally
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Calculation of number of genes
• (1/4)n= ratio of f2 individuals showing extreme phenotype
• n = (2n +1) phenotypic classes
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Statistical Analysis
• Mean (X) = ∑Xi/n• Variance (s2) = (∑Xi – X)2/n-1• Standard deviation (s) = √s2
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Heritabilty
• Genetic contribution to phenotypic variability
• Broad-sense Heritabilty (H2)• Narrow-sense Heritabilty (h2)• Quantitative trait loci (QTL)
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Broad-sense Heritabilty• H2 = proportion of
total variance caused by genetic variance
• H2 = 1.0, all genetic
• H2 = 0 all variation due to environment
• Vp= phenotypic var
• Vg= genetic var
• VE= enviromental var
• Vp= Vg + VE
• H2= Vg/Vp
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Calculation of H2(Broad-Sense)• VE= (3.1 + 3.9)/2
(4.7 + 3.5)/2VE = 4.1
• Vp = 47.7
• Vg = Vp – VE= 47.7-4.1
• H2= Vg/Vp
Mean Var
P1 short
40.4 3.1
P2 tall 93.7 3.9
F1 63.9 4.7
F2 68.7 47.7
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Estimation # of genes
• n = D2 / 8Vg• n = number of genes• D = difference of means of two parents• Vg = genetic variance• Assumes: alleles equal and additive,
assort independently, original parents are homozygous
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Narrow-sense Heritabilty
• Potential response to selection
• Additive variance• Dominance variance
• VG= VA + VD
• h2 = VA/ VE + VA + VD
• High h2 prediction of impact of selection in altering a population.
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Selection• h2 = M2- M/ M1 – M
response/selection differential
• M= mean of parental• M1= mean of selected
segment• M2= mean of offspring• Heritability low for traits
essential for survival
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Narrow-sense Heritabilty
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Twin Studies
H = (Vdz – Vmz)/ Vdz
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Quantitative trait loci (QTL)
• DDT resistance polygenic
• Each chromosome makes a contribution to survival.