Quantitative Genetics and Complex Trait Architecture
Variance components
Trait decomposition
Quantitative traits are shaped by many loci, environments, and their interactions. Phenotypic variance is commonly decomposed as , where genetic variance can be further partitioned into additive, dominance, and epistatic components.
Which component best predicts response to selection in many breeding and evolutionary contexts?
Additive effects are transmitted predictably from parents to offspring and therefore drive response to selection.
Correct answer: Additive genetic variance
Define heritability in one sentence.
Heritability is population- and context-specific, not a fixed property of a trait.
Correct answer: Heritability is the proportion of phenotypic variance attributable to genetic variation in a specified population and environment.
Polygenicity and prediction
Complex architecture
Most common human traits and disease risks are polygenic, with many variants of small effect and nonadditive interactions. Polygenic scores aggregate allelic effects across the genome, but portability and causal interpretation are limited by ancestry differences, LD structure, and environmental confounding.
Genetic variance components
Additive variance
- Main driver of narrow-sense heritability
- Most useful for prediction and selection
Nonadditive variance
- Includes dominance and epistasis
- Important biologically but harder to exploit in prediction
Why can polygenic scores perform worse when transferred across populations?
Differences in linkage disequilibrium, allele frequencies, and environment reduce portability.
Correct answer: Effect sizes and LD patterns differ across ancestries and environments
What is a genotype-by-environment interaction?
The same allele can have different effects across contexts.
Correct answer: A situation where the phenotypic effect of a genotype depends on the environment.
Interpretation warning
High heritability does not imply immutability, and low heritability does not imply no genetic influence.