Breeding Calculators

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Breeding is the controlled mating of organisms to improve desirable traits in offspring — harnessing the principles of genetics, quantitative genetics, and molecular biology to increase productivity, health, and adaptation. Plant breeding develops improved crop varieties; animal breeding improves livestock and companion animals. Key quantitative genetics concepts — heritability, selection differentials, breeding values, and response to selection — determine how rapidly traits can be improved across generations. Modern breeding increasingly integrates genomic selection, marker-assisted selection, and gene editing.

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Key Breeding Concepts

Heritability (h²)

The proportion of phenotypic variance in a population attributable to additive genetic variance:
h² = VA / VP = VA / (VA + VD + VI + VE)
Where VA = additive genetic variance; VP = total phenotypic variance. h² ranges 0–1; higher values indicate traits more responsive to selection.

Response to Selection

R = h² × S
Where S = selection differential (mean of selected parents − population mean) and R = response (change in trait mean in offspring). Example: dairy milk yield h² = 0.30, S = 500 kg → R = 150 kg per generation.

Breeding Value (EBV)

Expected Breeding Value = 2 × (deviation of progeny mean from population mean). Used to rank animals for selection. Genomic estimated breeding values (GEBVs) use SNP marker data to predict EBV with higher accuracy in young animals before progeny testing.

Selection Methods

  • Mass selection: Select individuals with best phenotype — most effective for high-h² traits
  • Progeny testing: Select based on offspring performance — better for low-h² traits and sex-limited traits
  • Family selection: Select whole families — used when individual records are unavailable or unreliable
  • Genomic selection: Uses genome-wide SNP markers to predict EBV — high accuracy, short generation interval

Inbreeding and Heterosis

  • Inbreeding: Mating related individuals → increased homozygosity → reduced performance (inbreeding depression)
  • Heterosis (hybrid vigor): Crosses between distinct lines or breeds often produce F₁ offspring that outperform both parents — exploits non-additive (dominance) genetic effects

Glossary

Heritability (h²)
The proportion of phenotypic variance attributable to additive genetic variance: h² = VA/VP. Ranges 0–1. Predicts response to selection: R = h² × S. Population-specific — varies with environment and management.
Selection Differential (S)
The difference between the mean phenotype of selected parents and the population mean: S = mean_selected − mean_population. Used in the breeder's equation: R = h² × S. Higher S = more intense selection = faster genetic gain.
Heterosis (Hybrid Vigor)
The tendency of F₁ hybrids from distinct lines or breeds to outperform their parents. Primarily due to dominance effects. Greatest for low-heritability traits. Exploited commercially in hybrid corn, broiler chickens, and crossbred livestock.

Frequently Asked Questions

Heritability (h²) = VA/VP — the proportion of phenotypic variance due to additive genetic variance. It predicts how well a trait responds to selection: R = h² × S. High h² (>0.5): milk fat %, weaning weight, litter size — responds well to mass selection. Low h² (<0.2): reproductive performance, disease resistance — requires progeny testing or genomic selection for effective improvement. h² is population-specific — varies with environment and population structure.

R = h² × S, where R = genetic gain per generation, h² = heritability, and S = selection differential (mean phenotype of selected parents − population mean). Example: corn grain yield h² = 0.50, average yield = 10 t/ha, top 20% selected parents average 12 t/ha, S = 2 t/ha → R = 0.50 × 2 = 1 t/ha improvement per cycle. Increasing S (selecting fewer, better animals) or h² (higher-h² traits) accelerates genetic gain.

Heterosis is the phenomenon where F₁ hybrids from crosses between distinct lines or breeds outperform the average of their parents (mid-parent heterosis) or even the best parent (best-parent heterosis). It results primarily from dominance effects — favorable dominant alleles masking recessive deleterious alleles that are exposed in inbred lines. Heterosis is greatest for low-heritability traits (reproduction, survival) and is commercially exploited in hybrid corn, broiler chickens, and pork production.

Genomic selection uses thousands to millions of SNP markers across the genome to predict an animal's or plant's Genomic Estimated Breeding Value (GEBV) without requiring progeny testing. A reference population with both genotype and phenotype data is used to train a statistical model; the model is then applied to young, genotyped-only individuals to predict their genetic merit. Benefits: high accuracy at young age; shorter generation intervals; applicable to sex-limited or difficult-to-measure traits. Transformed dairy cattle and aquaculture breeding since ~2009.