14 and 16 April, 2004

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Chapter 18
Quantitative Genetics
Continuous variation

• 14 and 16 April, 2004

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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.

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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.

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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

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Measures of central tendency
Mode most frequent class in sample bimodal two
frequent classes Mean arithmetic average
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Measures of dispersion

• Variance of sample average squared deviation
  from mean
• Standard deviation

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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

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R2.82
R2.99
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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

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

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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

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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 populations variation is
attributable to genotypic variation.
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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

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Meaning of H2

• It is a population parameter, with no application
  to particular individuals
• H2 gt 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

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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

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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

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

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