1. Chapter 7 Beyond alleles: Quantitative Genetics
Oldfield Mouse
Beyond alleles:
Quantitative Genetics
Evolution of Phenotypes
inland
dunes

2. Continuous traits have a complex genetic basis
Polygenic trait: influenced by many genetic loci
Human height, skin color
Quantitative genetics: study of the genetic mechanisms of continuous phenotypic traits
Start with phenotypic distributions in population; then look at how selection and other forces can cause frequencies to change
Population geneticists start with alleles at genetic loci and build the genotype and then the phenotype

3. Continuous traits have a complex genetic basis
Polygenic trait: influenced by many genetic loci
Interaction between alleles (epistasis)
Interaction with environment (phenotypic plasticity)
Quantitative genetics: study of the genetic mechanisms of continuous phenotypic traits

4. Hardy-Weinberg extended to polygenic traits
Loci Genotypes Phenotypes 1 2 3 3 9 27 3 5 7

5. Variance Mean = Variance =
How widely dispersed trait values are from the mean.
Mean =
Variance = n n
The more variation there is in a trait – the larger the variance for trait

6. Standard Deviation

7. Components of phenotypic variation
VP = VG + VE
Total phenotypic variance in population
Variance due to genetic differences
Variance due to environmental differences

8. Genetic and environmental influences create continuous distribution

9. Complex Phenotypic Traits
Vary continuously within population
Yields normal distribution of trait values around the population mean
Figure shows three hypothetical populations
Each has a mean phenotypic value of 0 and variances range from 1 to 3
As the variance of a sample increases, more and more individuals have trait values located far way from the mean

10. Complex traits vary continuously
Frequency
Distance from mean

11. Broad Sense Heritability
Proportion of phenotypic variance explained by genetic differences among individuals
Measures relative importance of genetic and environmental effects on trait expression

12. Problem
Broad sense heritability represents all genetic variance as a single value
In sexually reproducing organisms, not all genetic effects are transmitted to offspring
Some is lost during meiosis when each pair of chromosomes separates, associations between alleles breaks down
So - only some genetic variation actually contributes to the phenotypic resemblance between offspring and their parents.
Only this portion of the variance enables a population to evolve in response to selection.

13. Broad Sense Heritability
Combining VG and VE with only two loci with two alleles each creates a continuous phenotypic distribution
Bars show the five phenotypic trait values produced from additive combinations of alleles at two loci = VG
See figure 7.4
Small normal curve over each bar represents the distribution of phenotypes produced by each genotype caused by the environment
Small distributions overlap and the result approximates a smooth normal curve (blue line)
Combination of genetic and environmental effects yields a continuous distribution of phenotypes

14. Combining VG and VE with only two loci with two alleles each creates a continuous phenotypic distribution
Combining VG and VE with only two loci with two alleles each creates a continuous phenotypic distribution
Bars show the five phenotypic trait values produced from additive combinations of alleles at two loci = VG
See figure 7.4
Small normal curve over each bar represents the distribution of phenotypes produced by each genotype caused by the environment
Small distributions overlap and the result approximates a smooth normal curve (blue line)
Combination of genetic and environmental effects yields a continuous distribution of phenotypes
Small curves represent distribution of phenotypes produced by each genotype caused by the environment

15. Narrow sense heritability
Proportion of phenotypic variance explained by additive genetic variation
Causes offspring to resemble parents
h2 = VA / VP = VA / VA + VD + VI + VE
Additive
Dominance
Epistasis

16. Narrow Sense Heritability
Break down VG into smaller components
Additive effects
Dominance effects
Epistatic effects
Effect of an allele at one locus depends on which allele is present at another locus
Break VG into three sources of genetic variation (I = epistatic) Labrador color
VG = VA + VD + VI

17. Phenotypic Variation
VP = VG + VE
VP = VA + VD + VI + VE

18. Narrow Sense Heritability
VA is especially important = Proportion of phenotypic variance explained by additive genetic variation- have more genes in common
Causes relatives to resemble each other
Epistasis
Additive
Dominance

19. Narrow Sense Heritability
Why doesn’t H2 include Dominance or Epistasis?
Traits must be heritable for them to respond to selection
Heritability is the genetic component of phenotypic variance
Additive effects of alleles cause relatives to resemble each other, contribute to evolutionary response to selection
Dominance and epistatic effects of alleles are the interactions among alleles
Effect of an allele on the phenotype depends on what it is paired with - context dependent
Context breaks down each generation with meiosis

20. Narrow Sense Heritability
Estimate h2 for body mass of fish
Randomly pair fish and allow them to breed
Weigh parents and offspring
The more similar the offspring, the greater the Narrow Sense Heritability
Large fish produce large offspring
Plot offspring body size against average body size of parents
Slope of regression will yield quantitative estimate of h2 for the trait

21. Narrow Sense Heritability
Mean Parental Body Mass
Mean Offspring Body Mass
Slope = h2

22. Estimating heritability
Slope = h2

23. Narrow Sense Heritability
In asexual plants, animals, protists and bacteria
No meiosis, so no loss of context for an allele
Epistatic interactions between alleles have an important effect on progeny phenotypes
In highly inbred individuals, so many alleles are homozygous and identical by descent that interactions among alleles in offspring are likely to be the same as they were in the parents.
Use Broad Dense Heritability H2 to predict the response of the population to selection

24. Key Concepts
When components of variation are additive, genetic and environmental variance sum to total phenotypic variance
Heritability is the proportion of phenotypic variance due to genetic differences
Narrow sense heritability includes:
Additive effects
Dominance effects
Epistatic effects
Maternal/Paternal environmental effects

25. Modes of selection

26. Evolutionary Response to Selection
2 ways to calculate
Population geneticists – measure the strength of selection as the selection coefficient s
The amount, s by which the fitness of a genotype is reduced relative to the most fit genotype in the population
Quantitative geneticists measure selection for a trait as the difference in the mean of a trait of reproducing individuals and the mean of the trait for the general population (the selection differential, S)

27. Evolutionary Response to Selection
Selection without evolution
If differences in a trait is due solely to the environment, h2 = 0
Offspring will not resemble the parents
If all the differences in a trait are due solely to genetics (allele differences among individuals) h2 =1
Offspring track the parents regardless of environmental changes
Selection for body size means offspring in next generation will be larger. Strength of selection determines evolutionary response

28. Selection differential measures the strength of selection

29. Calculating the evolutionary response to selection The Breeder’s Equation
R = h2 x S
h2 x S reflects pheotypic variation that influences fitness (S), and the ability to transmit those phenotypic characteristics to offspring (h2)
As long as there is some selection. It can lead to a significant evolutionary change
These are exactly the points Darwin recognized as necessary for evolution in response to selection

30. Cumulative effects of directional selection can be large

31. Disruptive selection

32. Evolutionary response to selection
How much the population changes depends on:
Selection differential (S)
Heritability

33. High heritability results in larger change

34. Key Concepts
Selection on quantitative traits can take several different forms
Directional
Stabilizing
Disruptive
Evolution and selection are not the same
Selection can occur without evolution
The magnitude of change depends on:
Strength of selection (selection differential)
Heritability

35. Quantitative trait locus (QTL)analysis links traits with genes

37. QTL analysis of coat color in mice

38. QTL analysis of coat color in mice

39. Much of variation in coat color explained by differences in two genes
Corin also explains a small amount of variation

40. Expression of Agouti during development influences coat color

41. Genetic manipulation of dark mice makes them lighter

42. Genome-wide association mapping of natural populations groups individuals by phenotype

43. Key Concepts
QTL mapping identifies regions of the genome associated with phenotypic variation
First step toward elucidating genes responsible for phenotypic evolution

44. Environmental influences on quantitative traits
VP = VG + VE
Total phenotypic variance in population
Variance due to genetic differences
Variance due to environmental differences

45. Phenotypic plasticity
A single genotype produces different phenotypes depending on the environment

46. Reaction norm can predict response to environment

47. All genotypes may not respond to the environment in the same way
Genotype x environment interaction

48. Phenotypic plasticity in Caenorhabditis elegans

49. Plasticity can evolve

50. Rapid change can lead to mismatch between plastic traits and environment

51. Key Concepts
Differences in phenotypic plasticity may be heritable and can therefore evolve