Variation_genetic+model+for+quantitative+traits
Page 1: Genetic Model for Quantitative Traits
Page 2: Simply Inherited and Polygenic Traits
Simply Inherited Traits: Traits influenced by one or few genes.
Examples: coat color, presence of horns, genetic defects . ; (e.g., spider syndrome in sheep).
Phenotypes categorized as qualitative or categorical traits.
Minimal environmental influence on these traits.
Page 3: Polygenic Traits
Polygenic Traits: Traits influenced by many genes; no single gene has overriding effect.
Examples: growth rate, milk production, birth weight.
Generally quantified by numerical values; described as quantitative or continuous.
Exception: Dystocia, influenced by many genes but described categorically.
Environmental factors significantly affect polygenic traits.
Page 4: Basic Model for Quantitative Traits
Model Equation: P = µ + G + E
P: phenotypic value of an animal for a given trait.
µ: population mean for the trait.
G: genotypic value of the animal for the trait.
E: environmental effects on the phenotype.
Note: G and E expressed as deviations from population mean; sum of G and E in population equals zero.
Page 5: Genotypic Value
Genotypic Value (G): Overall effect of all genes on the phenotype of an animal.
Not directly measurable; expressed as: G = BV + GCV.
BV: Breeding value; GCV: Gene Combination Value.
Page 6: Breeding Value
Breeding Value (BV): Additive genetic value transmitted from parents to offspring.
Sum of individual gene effects, independent of dominance and epistasis effects.
Parental value significant for contribution of genes in next generation.
Animals selected based on estimated breeding values.
Page 7: Example of Breeding Value Calculation
Assume a trait affected by 5 loci with independent effects:
Allele impacts categorized by locus;
Example:
Locus 1: Average effects +3.0, -0.6 (sum +2.4)
Locus 2: +0.2, +4.2 (sum +4.4)
Resulting Breeding Value: +5.8
Page 8: Progeny Difference and Transmitting Ability
Progeny cache half of parental genes; this half is random.
Progeny Difference (PD) or Transmitting Ability (TA) defined as:
PD = TA = ½ BV
PD and TA indicate expected performance of offspring relative to population mean.
Page 9: Prediction of Progeny Difference
PD and TA not directly measurable but can be estimated through performance data.
Predicted values termed EPD (Expected Progeny Difference) or PTA (Predicted Transmitting Ability).
EPD used in beef, swine, sheep breeding; PTA in dairy breeding.
Page 10: Breeding Value of Offspring
Breeding value of offspring determined by additive effects from both sire and dam:
Example: Sire BV +2.5 kg, Dam BV +1.5 kg leads to average expected offspring BV of +2.0 kg.
If population mean weaning weight: 18 kg, expected offspring weight = 20 kg.
Page 11: Gene Combination Value (GCV)
GCV: Effect of gene combinations (dominance and epistasis); not transmittable to offspring.
Not important for selection as it cannot be passed on.
Page 12: Example of Gene Combination Value
Locus affecting litter size in swine with alleles T (dominant) and t (recessive).
Independent effects result in different BV values for different genotypes:
TT, Tt, tt genotype effects summarized in table format.
Page 13: Producing Ability (PA)
Producing Ability: Performance potential for repeated traits over time.
Key for understanding how genetic and environmental factors permanently affect performance.
Consideration of genotypic value and permanent environmental effects together.
Page 14: Permanent vs. Temporary Environmental Effects
Permanent Environmental Effects (Ep): Long-lasting factors affecting performance.
Examples: early nutrition, permanent udder problems affecting milk production.
Temporary Environmental Effects (Et): Short-term factors, change over time.
Examples: forage quality, weather conditions affecting performance inconsistently.
Page 15: Model for Producing Ability
Producing Ability equation: PA = BV + GCV + Ep
Population mean of PA typically averages zero.
Overall model for repeated traits: P = µ + BV + GCV + Ep + Et
Page 16: Example: 305-d Milk Production Records
Example of performance records for two cows showing P, BV, GCV, Ep, Et contributions for each.
Calculated PA shows negative and positive values indicating different productive capacities.
Page 17: Importance of Producing Ability
Critical for commercial producers assessing productive capacity.
Dairy farmers adjust feed based on producing ability.
Predicted PA termed Most Probable Producing Ability (MPPA); formula given for predicting next record.