Quantitative Genetics

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Quantitative Genetics

• julian_christians_at_sfu.ca
• See also Chapter 20 in
• Griffiths et al.
• Introduction to Genetic Analysis

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Quantitative Genetics

• Continuous phenotypic variation within
  populations
• Whole organism level
• Causes of variation
• Genes vs. environment
• Interactions between genes and environment
• Components of genetic variation
• Components of environmental variation

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Why is quantitative genetics important?

• Medicine
• Disease variation
• Complex disorders
• caused by multiple genetic and environmental
  factors
• Understanding genetic vs. environmental causes
• prevention
• genetic counselling
• genetically-tailored treatments

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Why is quantitative genetics important?

• Agriculture
• Economically important traits quantitative
  traits
• Quantitative genetics theory -gt basis for
  breeding programmes
• Environmental variation reduces efficiency of
  selection

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Why is quantitative genetics important?

• Consequences of inbreeding and outcrossing
• Agriculture inbred lines, hybrids, F1s
• Conservation endangered species, captive
  breeding programmes

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Why is quantitative genetics important?

• Evolution
• Natural selection requires heritable variation
  for traits
• What are the forces that maintain variation
  within populations?
• Balance between selection, drift and mutation
• Balancing selection?

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Why is quantitative genetics important?

• Evolution
• Does evolution proceed by small steps or big
  jumps?
• What is the relative importance of pre-existing
  variation vs. new mutations?
• Do genetic correlations between traits pose
  constraints on evolution?

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History

• Around 1900, there were two camps
• Biometricians
• Continuous traits
• Mendelians
• Discrete traits

Are discrete traits inherited in the same way as
quantitative traits?
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History

• Reconciliation
• Multiple loci (genes) contribute to variation!

Is variation caused by a few loci of large
effects or many loci with small effects?
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Heritability
Measured using resemblance between relatives
h2 genetic variation phenotypic variation
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• QTL
• quantitative trait locus
• region of chromosome that contributes to
  continuous variation

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Questions

• Is variation caused by a few loci of large
  effects or many loci with small effects?
• To what extent do individual genes affect
  variation in different traits?
• Are the same genes responsible for variation in
  different populations/ species?
• What are the genes responsible for variation and
  adaptation?

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phenotype
genetic markers
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Basis of QTL mapping
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Statistical Support
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Problems

• Mapping resolution is limited by recombination,
  which is limited by the number of individuals in
  cross or pedigree
• Small effects are difficult to detect

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Approaches

• Controlled crosses of inbred or outbred lines
• Pedigrees
• Population-wide

Suitable for natural populations
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Statistical Support
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My work QTL affecting size in mice
Statistical Support
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PAPP-A2

• enzyme that cleaves an insulin-like growth factor
  binding protein (IGFBP-5)
• IGFBPs modulate the effects of insulin-like
  growth factors

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Interested in studying the physiological and
genetic mechanisms underlying variation within
species?Contact me!julian_christians_at_sfu.ca