Chapter 21

  • Complex traits are influenced by multiple genes, interactions, variation in environment, and interaction between alleles and environment
  • Analyzing quantitative trait variation
    • Factors causing continuous variation of quantitative traits- number of genes that determine the trait and genetic and environment factors that affect penetrance and expressivity of genes
    • Goal of quantitative effects from the environmental effects
  • Studies of dandelions can help sort out the effects of genes vs the environment
    • Goal is to compare influence of genes and environment on length of stem at flowering
  • Environmental Variance
    • most dandelion seeds arise from mitotic divisions (all seeds from single plant are genetically identical)
    • grow in variable environment, such as a hillside
  • Genetic Variance
    • to calculate genetic variance- keep environment constant by growing genetically different seeds in a greenhouse
  • Total Phenotypic Variance
    • grow genetically diverse seeds on a hillside
    • total phenotype variance= genetic variance + environmental variance
    • wider graph
  • Heritability- proportion of phenotypic variance due to genetic variance
    • Heritability of a trait is the porportion of total phenotypic variance to genetic variation (VG/VP)
    • broad sense variability is measured only when comparing identical twins to each other
  • Equations
    • VP = VE + VG
    • VG = VA + VD + VI
    • VA = Variance due to additive genetic effects
    • VD = Variance due to dominance effects
    • VI = Variance due to loci interaction
  • Narrow-sense
    • VA/VP
    • VA/(VA+VD+V1+VE)
    • narrow sense heritability looks at the additive variance
    • genetic relatedness- average fraction of common alleles at all genetic loci shared by relatives
    • only 1 allele at any given locus is shared
  • Relationship between heritability and mean phenotypic values of parents, offspring, and overall population
  • Measuring the heritability of bill depth in populations of Darwin’s finches
    • Geospiza fortis on Daphne Major in the Galapagos Islands
    • correlation between beak size of offspring and the average of parents’ beak size
  • Results in finch populations had no environmental or genetic effects
    • if environment had no effect- heritability would be 1
    • if no genetic contribution- heritability would be 0
  • Concordance of a trait in two children raised in the same family
    • heritability 0- no differences observed between monozygotic, dizygotic, or unrelated by adoption
    • if heritability is 1- differences would be observed in monozygotic, dizygotic, or unrelated by adoption
  • A trait’s heritability determines its potential for evolution
    • selection differential (S)- difference between value for this trait in the parents and value for this trait in the entire population (breeding and non-breeding)
    • Response to selection (R)- amount of change in the mean value of a trait that results from selection
    • R=h^2S
  • Two approaches for mapping QTLs
    • Quantitative trait loci- genes that contribute to complex traits
    • mapping QTLs depends on producing individuals with different genetic compositions
    • Direct QTL mapping- requires crosses between individuals that differ in the phenotype of interest
    • Association mapping- takes advantage of the hisotry of a random breeding population
  • Direct QTL mapping- looks for joint segregation of the phenotype and genetic markers
    • rough mapping:
    • cross individuals showing 2 extremes of a phenotype
    • identify DNA markers that segregate with phenotype
    • markers strongly correlated with phenotype are likely to be near genes that influence the trait
  • Rough mapping- involved in tomato fruit size
    • start with isogenic large (l) and small (p) tomato strains
    • BC1 generation has many sizes due to variation at many genes
    • at each gene, the backcross generation consists of p/l and l/l individuals
  • QTLs identified based on correlation with phenotype
    • genotyped and measured weight of hundreds of tomatoes
    • differences in mean weight of l/l and p/l for a given marker indicates the marker is linked to a QTL
  • Genome-Wide Association Studies have identified genes that control only a fraction of the total trait variation
    • most variation has occurred recently due to population growth
    • recent variation hasn’t spread widely around the globe
    • acquisition of huge amounts of data is necessary