Beyond Alleles: Quantitative Genetics and the Evolution of Phenotypes

Beyond Alleles: Quantitative Genetics and the Evolution of Phenotypes

Student's Learning Objectives

  • Define polygenic traits
  • Define quantitative genetics
  • Describe the various components of phenotypic variance
  • Explain the difference between broad sense and narrow sense heritability
  • Describe one method of estimating narrow sense heritability
  • Important Dates:
    • Quiz 2: Chapters 4, 5, and 6 – Due: Oct 7
    • Test 2: Chapters 4, 5, and 6 – Dates: Oct 21 & 22

Polygenic Traits

  • Definition: Polygenic traits are influenced by many genetic loci.
    • May be affected by interactions between alleles at different loci, known as epistasis.
    • Polygenic traits typically exhibit continuous variation among individuals, as seen in traits like human height.

Quantitative Genetics

  • Definition: Quantitative genetics is the study of the genetic mechanisms and the evolution of continuous phenotypic traits.
  • Example of a Polygenic Trait:
    • Height in humans exemplifies a polygenic trait, demonstrating a range rather than distinct classifications (e.g., tall vs short).
    • In contrast, single gene traits, such as pea plant seed texture (round vs. wrinkled) or color (green vs. yellow), appear in one or two distinct phenotypes.

Variance of a Polygenic Trait

  • Variance: A statistical measure that describes the dispersion of trait values around their mean.
    • It quantifies the amount of variation for a trait within a population.

Example of Variance

  • Three hypothetical populations with a mean phenotypic value of 0:
    • Population with the tallest curve has a variance of 1.
    • Population with the lowest curve exhibits a variance of 3.

Questions on Variance

  • What is variance?
    • A statistical measure showing the average value in a population.
    • A statistical measure of how a gene responds to the environment.
    • A measure of the dispersion of trait values about their mean.
    • The pattern of phenotypic expression of a single genotype across a range of environments.

Components of Phenotypic Variation

  • Total Phenotypic Variance (V_P) is composed of:
    • Genetic Variance (V_G)
    • Environmental Variance (V_E)
    • Formula: VP = VG + V_E

Notes on Phenotypic Variation

  • In situations where environmental factors contribute more to phenotypic variation than genetic factors:
    • If VE > VG
  • Conversely, when environmental influences are negligible:
    • If VP ext{ } ext{is approximately } VG

Heritability

Broad Sense Heritability (H²)

  • Broad sense heritability is defined as the proportion of the total phenotypic variation in a trait that is attributable to genetic differences among individuals.

Narrow Sense Heritability (h²)

  • Narrow sense heritability is the proportion of phenotypic variance explained by the additive effects of alleles.
    • Components:
    • V_A: Additive genetic variance (contributes to offspring resemblance to parents).
    • V_D: Dominance effects of alleles on variance.
    • V_I: Variance due to epistatic interactions (allelic interactions between different loci).
  • Note: Allelic interaction effects are not inherited by offspring and do not influence resemblance between parents and offspring due to genetic recombination.

Offspring-Parent Regression

  • A method for estimating narrow sense heritability:
    • An example entails assessing the body mass of fishes:
    • Pair fish for breeding and weigh both parents and offspring.
    • Plot the offspring's body size against the average size of their parents, referred to as the midparent value.
    • The slope of the regression line indicates the narrow sense heritability.
    • Higher narrow sense heritability values suggest that offspring share more traits with their parents.

Examples of Estimating Narrow Sense Heritability

  • Shown in graphical data (A, B, C, D):
    1. Crassostrea gigas (Mean offspring pigmentation): Slope h² = 0.59
    2. Offspring mean ripening date: Slope h² = 0.94
    3. Tachycineta bicolor (Offspring mean tarsus length): Slope h² = 0.50
    4. Raphanus raphanistrum (Offspring mean pistil length): Slope h² = 0.74
    • Interpretation of Figure D: 74% of variation in pistil length is attributable to genetic diversity; 26% is due to environmental differences.

Summary Question

  • What does an offspring-parent regression show?
    • Broad sense heritability.
    • Frequency of alleles in the population.
    • How closely offspring trait values resemble traits of their parents.
    • The impact of genetic drift on the offspring's phenotypes within the population.