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Genetics of Quantitative Traits

Genetics of Quantitative Traits. Quantitative Trait. Any trait that demonstrates a range of phenotypes that can be quantified Height Weight Coloration Size. Continuous Variation vs Discrete Phenotypic Classes. Continuous variation

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Genetics of Quantitative Traits

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  1. Genetics of Quantitative Traits

  2. Quantitative Trait • Any trait that demonstrates a range of phenotypes that can be quantified • Height • Weight • Coloration • Size

  3. Continuous Variation vs Discrete Phenotypic Classes • Continuous variation • Offspring show a range of phenotypes of intermediate range relative to the parental phenotype extremes • Discrete classes • Offspring show phenotype exactly like either parent (dominance/recessiveness) • or in a single intermediate class (incomplete dominance) • or have a combinatorial phenotype (co-dominance)

  4. Example of Continuous Variation

  5. Demonstrating Genetic Control of Variation • Individually cross F2 at phenotypic extremes • Subsequent ranges of progeny are centered on F2 phenotype

  6. Polygenic Inheritance • A trait controlled by multiple genes with additive and non-additive allele types • Additive allele (Uppercase) • an allele which contributes to the observe phenotype • causes more color, height, weight, etc.. • Non-additive allele (lowercase) • an allele which does not contribute to observed phenotype • causes less color, height, weight, etc…

  7. Polygenic Control of Wheat Color P F1

  8. Wheat Color Defined by Two Genes • A and B are additive alleles of two genes • a and b are non-additive alleles of the same two genes • The number of additive and non-additive alleles in each genotype defines a distinct phenotype • 4 additive alleles ------ AABB • 3 additive alleles ------ AaBB, AABb, • 2 additive alleles ------ aaBB, AAbb, AaBb • 1 additive allele ------- Aabb, aaBb • 0 additive alleles ------ aabb • Give 5 phenotype classes

  9. How Many Genes Control a Trait? & How Many Phenotypes are Possible?

  10. Statistics Numbers of individuals with that phenotype Range of the phenotype being measured

  11. Mean (aka Average) and Variance Height of Population 2 • These two populations have a mean height that is the same • The range of heights in each population is quite different Number of Individuals with Indicated Height Height of Population 1 1ft 2.5ft 7.5ft 10ft (Height)

  12. Measuring the Variance i=1 (Xi - X)2 n s2 = n-1 s = s2 s = SX  n • Sample variance s2 • Standard deviation = square root of variance • Standard error n = # of individuals for which trait has been quantified

  13. Weight Distribution of F1 & F2 TomatoProgeny

  14. Example Statistics Problem Mean: XF1 = 12.04 Mean: XF2 = 12.11 Variance: s2F1 = 1.29 Variance: s2F2 = 4.27 Stnd Dev: sF1 = 1.13 Stnd Dev: sF2 = 2.06 12.04 ± 1.13 12.11 ± 2.06 See table 6.4 (4th ed) or table 5.4 (3rd ed)

  15. Nature or Nurture • Phenotypic variation due to genetic factors • Phenotypic variation due to environmental factors • Heritability • Broad-sense • Measure of variance due to genetics vs environment • Narrow-sense • Measure of selectability

  16. Identifying Environmental vs Genetic Factors Influencing Variability • Inbred strains • an inbred population is highly homozygous • lethal recessives are lost • allele frequencies are stabilized • Variation in inbred populations in differing environments is due to environmental factors – VE • Variation in inbred population in same environment is due to genetic differences - VG

  17. Environmental vs Genetic Factor Measurement • If extreme phenotypes of highly inbred line are selected, do F1 show deviation from P mean? • yes – variance is genetic • no – variance is environmental

  18. Broad-sense Heritability VG H2 = VP • Heritability index – H2 Proportion of variance due to genetic factors • VP = phenotypic variance (ie s2 for a measured trait in a population) • VP = VE + VG • VG = genetic variance • VE = environmental variance

  19. Narrow-sense Heritability R S h2 = • S = deviation of selected population mean from whole population mean • R = deviation of offspring mean from whole parental population mean • ratio of R to S describes narrow-sense heritability – ie how selectable is the trait h2 near 1 means trait could be altered by artificial selection

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