Overview
This lecture covers key quantitative genetic methods for estimating heritability, focusing first on laboratory breeding designs, then introducing a more advanced method for wild populations, and discussing the challenges posed by environmental effects.
Methods to Estimate Heritability in the Lab
- Quantitative genetics aims to measure resemblance among relatives to estimate additive genetic variance and heritability.
- Parent-offspring regression quantifies resemblance between parent traits and offspring traits using linear regression; the slope estimates narrow-sense heritability.
- Example: In birds, a regression slope of 0.5 for tarsus length means heritability is 0.5 (50% of variance is additive genetic).
- Paternal half-sibling analysis examines trait similarity among offspring from one father and different mothers to isolate additive genetic effects.
- Full siblings share both genetic and environmental factors, making it hard to separate genetic from environmental effects.
- Paternal half-siblings share only paternal genes, and usually grow up in different environments, so similarities are genetic.
- A formal paternal half-sib breeding design (nested design) involves mating each sire to multiple dams and measuring offspring traits.
- Analysis of variance (ANOVA) partitions trait variance into components: sire (genetic), dam (maternal), and within-family (error).
- Additive genetic variance (VA) is estimated by multiplying the sire component by four.
- Total phenotypic variance (VP) equals the sum of sire, dam, and within-family variances.
- Narrow-sense heritability (h²) is calculated as VA divided by VP.
Advanced Breeding Designs
- Cross-classified (factorial) breeding designs, like the North Carolina II design, cross each sire with multiple dams and vice versa.
- This design allows estimation of both paternal and maternal genetic effects, and non-genetic maternal influences.
- The difference between paternal and maternal components reveals maternal environmental effects (e.g., egg size).
- Interaction between sire and dam effects can indicate genetic compatibility between parents.
- This design works best with externally fertilizing species where eggs and sperm can be split among multiple crosses.
- It's especially useful for fish and other broadcast spawners.
Key Terms & Definitions
- Parent-offspring regression — A method using regression to estimate heritability by comparing parent and offspring traits.
- Paternal half-sibling design — Breeding design to estimate genetic variance by comparing offspring from one sire with multiple dams.
- Additive genetic variance (VA) — Genetic variance attributed to additive effects of genes.
- Total phenotypic variance (VP) — Overall variance in a trait within a population.
- Narrow-sense heritability (h²) — Proportion of phenotypic variance due to additive genetic variance (h² = VA / VP).
- Nested design — Experimental design where dams are nested within sires.
- Cross-classified design — Design where multiple sires and dams are crossed in all combinations.
- Maternal effect — Non-genetic influence of the mother on offspring traits.
Action Items / Next Steps
- Read suggested material for deeper understanding of quantitative genetic designs and their underlying assumptions.
- Prepare for the next segment on methods for estimating quantitative genetic parameters in natural populations.