In-Depth Notes on Heritability

Understanding Heritability

  • Definition: Heritability refers to the degree to which genetic factors contribute to the variation in a specific trait within a particular population at a specific time.

Key Concepts of Heritability

  • Proportion of Variation

    • Heritability (h²) quantifies the portion of trait variation attributable to genetics.

    • Scale: 0 (no genetic influence) to 1 (total genetic control).

  • High vs Low Heritability

    • High heritability (h² > 0.4 or 40%): Breed for genetic gain.

    • Low heritability (h² < 0.15 or 15%): Manage traits, expect slow genetic gain.

Methods to Calculate Heritability

  1. Broad-Sense Heritability:

    • Formula: h2 = Vg/Vp

    • More complex calculations, detailed in higher courses like GENE301.

  2. Mean Trait Values:

    • Formula: h^2 = \frac{R}{S}

    • R = response to selection, S = selection differential.

  3. Parent-Offspring Regression:

    • Graph the relationship between parent and offspring trait values.

    • Best fit slope indicates heritability;

      • slope of 0 means no genetic influence

      • slope of 1 indicates total genetic control.

    • Important for breeding applications.

Example of Parent-Offspring Regression

  • Illustration: For a given trait (like beak depth in sparrows):

    • Regression equation example: y = 0.98x - 0.01 where y is the offspring trait value, and x is the parent trait value. This high slope indicates a strong genetic correlation between the parents' and offspring's beak depths, suggesting significant heritability for this trait.

    • Coefficient of determination (r²) value indicates variance accounted for (e.g., r² = 0.496). This suggests that nearly 49.6% of the variance in the trait can be attributed to genetic factors.

Factors Affecting Heritability

  • Environmental consistency is crucial for reliable heritability estimates.

    • Variations in environment (e.g., different orchards) may affect results.

DETERMINING A HERITABILITY VALUE FOR TEMPERAMENT

  • Steps for Assessing Heritability of Traits (e.g., Docility):

    1. Define the Trait: Specific behaviors for temperament. นิสัยโดยธรรมชาติ

    2. Develop a Temperament Scale: Use observational studies and behavioral tests.

    3. Data Collection: Gather quantitative temperament data from both parents and offspring.

    4. Selection: Choose best individuals to breed.

    5. Data Collection on Progeny: Again measure temperament in offspring.

    6. Heritability Calculation: Use methods like h^2 = \frac{R}{S} or regression.

Livestock Heritability Estimates

  • Sheep and cattle have varying heritability across traits:

    • Sheep:

    • Birth Weight: 0.3

    • Growth Rate: 0.3

    • Mature Body Weight: 0.4

    • Cattle:

    • Birth Weight: 0.45

    • Growth Rates: Range from 0.3 to 0.46

  • Notably, genetic similarities can be very high (>99%) despite differing heritability values.

Practical Example of Genetic Gain Calculation

  • Average growth rate of lambs: 150 g/day.

    • Selected ram and ewe lambs benchmarks: 300 g/day (rams) and 250 g/day (ewes).

    • Compute superiority:

    • Ram superiority: 300 - 150 = 150 g/day

    • Ewe superiority: 250 - 150 = 100 g/day

    • Average superiority: (150 + 100)/2 = 125 g/day.

    • Expected genetic gain with h^2 = 0.3:

      • 0.3 x 125 g/day = 37.5 g/day

    • Resultant expected growth rate for offspring: 187.5 g/day.

Considerations in Breeding Practices

  • Challenges:

    • Genetic gains across generations may show variation and slower improvement due to risks like inbreeding.

Conclusion

  • Understanding heritability is crucial for effective breeding.

  • Heritability estimates are context-specific and can be impacted by a range of factors, including population dynamics and environmental conditions.