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Chapter 18

Chapter 18. Quantitative Genetics. 13 - 18 April, 2005. Continuous variation. Overview. In nature, variation in most phenotypic characters is continuous. Such quantitative variation is studied with statistical techniques.

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Chapter 18

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  1. Chapter 18 Quantitative Genetics 13 - 18 April, 2005 Continuous variation

  2. Overview • In nature, variation in most phenotypic characters is continuous. • Such quantitative variation is studied with statistical techniques. • Continuous variation may be the result of segregation of interacting alleles at several loci with cumulative effect on phenotype. • Environmental interaction with genotype contributes to phenotypic variance. • Heritability is a population trait, not an individual one.

  3. Quantitative genetics • Genetics of continuously varying characters • Attempts to determine genetic variation contributing to character • number of loci with segregating alleles • how genes interact with one another and the environment (norm of reaction) • How do loci interact (dominance, epistasis, etc) • possible maternal effects • Often requires statistical analysis • Quantitative traits: small variation between genotypes, large variation within genotype.

  4. Statistical measures • Statistical distribution: description of set of quantitative measurements • graphical representation: e.g, histogram • distribution function: continuous curve • Mean: measure of central tendency (average) • mode: most frequent observation • Variance: measure of dispersion about mean • Correlation: relationship between two measured quantities

  5. Measures of central tendency • Mode: most frequent class in sample • bimodal: two frequent classes • Mean: arithmetic average

  6. Measures of dispersion • Variance of sample: average squared deviation from mean • Standard deviation

  7. Measures of relationship (1) • Correlation coefficient: relation between two variables, x and y • calculated from covariance of sample and standard deviations of x and y

  8. R2=.82 R2=.99

  9. Measures of relationship (2) • Correlation is estimate of precision of relation between two variables; can not be used to predict value of one given the other. • Regression describes relationship between two variables and allows their prediction. linear regression:

  10. Although most quantitative traits are polyfactorial, variation at a single locus can behave quantitatively.

  11. Norm of reaction • The relationship between environment and phenotype for a given genotype • Studied by subjecting homozygous lines to different environments • replicated (e.g., cloned) genotypes • inbred lines (e.g., from repeated sib mating) • use of dominant markers and crossover suppressors to make lines homozygous for particular chromosomes • Studies indicate that phenotypic differences between genotypes are small

  12. Heritability • Important to know whether genes contribute to phenotypic variation of quantitative character • Heritability is a population trait, not property of individual • Not same as familial trait shared by members of a family • Characters are heritable only if similarity arises from shared genotypes • estimated from phenotypic similarity of relatives • estimated by cosegregation of gene markers • difficult to estimate in humans

  13. H2: broad heritability s2p = total phenotypic variance in population s2g = genetic variance s2e = environmental variance cov = covariance between genotypic and environmental effects s2p = s2g + s2e + 2 cov ge H2 tells what part of population’s variation is attributable to genotypic variation.

  14. Estimating H2 • Intercross homozygotes and measure variance within each heterozygous genotype. Average is s2e which can be subtracted from s2p to give s2g. • Statistically estimate from genetic similarities between relatives, particularly using difference in phenotypic correlation between monozygotic and dizygotic twins

  15. Meaning of H2 • It is a population parameter, with no application to particular individuals • H2 > 0 means that genetic variance is present • H2 = 0 means that there is no genetic variation; genes may still be relevant to trait (as in development) • Value of H2 provides limited prediction about effect of environmental modification • H2 is specific to environmental conditions under which it is measured

  16. Locating genes • Difficult to identify genes for quantitative characters • Candidate gene: from prior biochemical or developmental knowledge • Quantitative trait loci (QTL) may be located to regions of chromosomes by cosegregation with marker genes • Dominance and epistasis may be involved

  17. h2: narrow heritability • Genetic variance can be divided into additive genetic variance and dominance variance • h2 is useful in determining whether there is selectable genetic variation • useful in plant and animal breeding

  18. Assignment: Concept map, Solved Problems 1-3, All Basic and Challenging Problems.

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