Ch 19b: Genetic Analysis of Quantitative Traits: Calculating Heritability

Genetic Analysis of Quantitative Traits

Introduction to Quantitative Traits

Quantitative traits are those that show continuous variation, such as height or eye size, and are typically influenced by multiple genes and environmental factors. The study of these traits involves understanding how genetic and environmental variation contribute to phenotypic variation.

• Quantitative traits: Traits that vary continuously and are usually polygenic.

• Phenotypic variation: Observable differences in a trait among individuals in a population.

Heritability: Measuring Genetic Contribution to Phenotypic Variation

Broad Sense Heritability

Heritability quantifies the proportion of phenotypic variation in a population that is attributable to genetic variation. Broad sense heritability considers all genetic variance components.

• Phenotypic variance ($V_P$): Total variance observed in a trait.

• Genetic variance ($V_G$): Variance due to genetic differences.

• Environmental variance ($V_E$): Variance due to environmental factors.

Key Equations:

• $V_P = V_G + V_E$

• Broad Sense Heritability: $H^2 = \frac{V_G}{V_P}$

Interpretation:

• If all phenotypic variation is genetic: $V_P = V_G \rightarrow H^2 = 1$

• If all phenotypic variation is environmental: $V_P = V_E \rightarrow V_G = 0$ and $H^2 = 0$

• If both genetic and environmental factors contribute: $V_P = V_G + V_E \rightarrow 0 < H^2 < 1$

Example: Heritability in Blind Cavefish

Heritability can be estimated by measuring trait variance in genetically homogeneous and heterogeneous populations.

• In cavefish, eye size variance ($V_P$) is measured in different populations.

• Genetically homogeneous population: $V_P = V_E$

• Genetically variable population: $V_P = V_G + V_E$

Calculation:

• $V_P = 0.563$ cm2, $V_E = 0.057$ cm2

• $V_G = V_P - V_E = 0.563 - 0.057 = 0.506$ cm2

• $H^2 = \frac{V_G}{V_P} = \frac{0.506}{0.563} = 0.899$

Conclusion: 90% of the phenotypic variation in eye size is due to genetic variation.

Using Twins to Study Heritability

Monozygotic vs. Dizygotic Twins

Twin studies are a classic approach to estimate heritability by comparing trait variance between identical (monozygotic) and fraternal (dizygotic) twins.

• Monozygotic (MZ) twins: Share nearly all genetic material; $V_P = V_E$

• Dizygotic (DZ) twins: Share about 50% of segregating alleles; $V_P = V_E + (1/2)V_G$

Estimating Genetic Variance:

• $V_G = 2(V_P^{DZ} - V_P^{MZ})$

Estimating Heritability:

• $H^2 = \frac{V_G}{V_P^{DZ}}$

Worked Example: Height in Twins

• Variance in MZ twins: $V_E = 15$ cm2

• Variance in DZ twins: $V_P = 25$ cm2

• $V_G = 2(V_P - V_E) = 2(25 - 15) = 20$ cm2

• $H^2 = \frac{20}{25} = 0.80$

Interpretation: 80% of the variance in height is due to genetic factors.

Limitations of Twin Studies

• Shared maternal effects: MZ twins may have more similar environments than DZ twins, inflating $V_E$ for DZ twins.

• Shared treatment: MZ twins often treated more similarly than DZ twins.

• Genetic differences: DZ twins may have more genetic differences, increasing $V_E$.

Concordance Rates in Twin Studies

Percent Concordance Table

Concordance rates measure the percentage of twin pairs that both express a trait. Higher concordance in MZ twins compared to DZ twins suggests a genetic component.

Disease/Trait	MZ Twins (%)	DZ Twins (%)
Alzheimer disease	60	25
Autism	70	10
Schizophrenia	60	20
Depression	70	25
Congenital hip dislocation	35	3
Insulin-dependent diabetes	50	10
Intellectual disability	70	45

Key Points:

• MZ > DZ concordance indicates genetic contribution.

• High concordance in both MZ and DZ twins suggests environmental factors.

Additional info: The notes also reference the Minnesota Twin Study, a landmark study in behavioral genetics, and discuss the importance of considering both genetic and environmental influences when interpreting heritability estimates.