Final Exam Slides

• Quantitative genetics


  The study of complex traits that are influenced by multiple genes and the environment
• Complex traits do not follow simple rules of Mendelian inheritance due to involvement of multiple genes and impact of the environment
• Types of complex characteristics

  • Continuous
  • Meristic
  • Threshold
• Continuous characteristics


  Vary across any value, limited only by how well we can measure it
• Continuous characteristics


  • Height

  
• Meristic characteristics


  Can have complex (genetic and environmental) determinants, but have a limited number of distinct phenotypes
• Threshold characteristics


  Have only two phenotypes (traits), but the liability for developing the trait varies continuously
• Phenotype depends on the number of alleles for a certain gene in incomplete dominance and additive alleles
• Incomplete dominance and additive alleles

  • Dark red-grained wheat crossed with white-grained wheat resulted in intermediate color in F1 and 7 distinct phenotypic classes in F2

  
• Polygenic characteristic


  One that varies due to different alleles at multiple genes
• If alleles are equally additive and there is no environmental influence, then for polygene # = n, the frequency of extreme phenotypes in F2 = 1/4n and the number of phenotypic classes = 2n + 1

  
• Polygenic characteristic


  • Corolla length in flowers
• East's analysis of 444 F2 plants for corolla length suggested 5 or more genes are likely involved

  
• Sample


  A measure of a subset of the individuals in a population
• Normal distribution


  A symmetrical distribution often called a "bell-curve"
• Mean (x̄)


  The average value for a set of numbers
• Variance (s^2)


  A measurement of how spread out a distribution is, the average squared deviation from the mean
• Standard deviation (s)


  Often used to describe variability, related to variance
• In a normal distribution, 66% of measurements will lie within one standard deviation of the mean, 95% within two standard deviations, and 99% within three standard deviations
• Heritability


  A measure of the proportion of phenotypic variation that is due to genetic differences

  
• Multiple factor hypothesis


  An individual's measurement of a trait (T) is the sum of the population mean (μ), the deviation due to genetic factors (g), and the deviation due to environmental factors (e)
• Analysis of variance


  VP = VG + VE, where VP is the total phenotypic variance, VG is the genetic variance, and VE is the environmental variance
• Quantitative genetics is the study of complex, or quantitative, characteristics that display a continuous range of variation and do not behave in a simple Mendelian fashion
• Announcements


  • Homework due today @ 11:59pm
  • Lab report re-writes due May 3rd @ 11:59pm
  • Senior celebration this Friday – RSVP!
• Learning objectives


  • Calculate broad- & narrow-sense heritability using correlation, regression, and the breeder's equation
  • Describe some of the limitations of heritability
  • Describe the process of quantitative trait locus (QTL) mapping

  
• Complex, or quantitative, characteristics display a continuous range of variation and do not behave in a simple Mendelian fashion
• Continuous, meristic, or threshold traits are often polygenic and/or influenced by environmental factors
• Frequency of extreme phenotypes for a polygenic characteristic

  1/4^(# of genes)
• Number of phenotypic classes for a polygenic characteristic

  2n + 1, where n is the number of genes
• Describing the phenotypic distribution of a population


  Using frequency distributions & summary statistics (mean, variance, standard deviation)

  
• Phenotypic variance (VP)


  Genetic variance (VG) + Environmental variance (VE)
• Heritability


  A measure of the proportion of phenotypic variation that is due to genetic differences

  
• Quantitative Genetics


  • Defining quantitative characteristics
  • Analyzing polygenes
  • Using distributions
  • Using correlations & regressions
  • Quantitative trait locus (QTL) mapping
• Heritability is a measure of the proportion of phenotypic variation that is due to genetic differences

  
• Analysis of variance (VP)

  VP = VG + VE + VGE, where VP is total phenotypic variance, VG is genetic variance, VE is environmental variance, and VGE is genetic-environmental interaction variance
• Eliminating components of variance

  1. Eliminate VE (and VGE) by growing in controlled environment, then VG = VP

  2. Eliminate VP by raising genetically identical individuals, so VP = VE
• Broad-sense heritability (H^2)


  The proportion of phenotypic variance due to genetic differences in a population

  
• Broad-sense heritability (H^2) will always be between 0 and 1
• Additive genetic variance (VA)


  Variance due to genes that act additively to genotype
• Dominance genetic variance (VD)


  Variance due to alleles that act dominantly