In-Depth Notes on Heritability

Heritability (h²) Overview

  • Definition: Heritability describes the extent to which genes control the expression of a trait in a population.

Key Concepts of Heritability

  • Application:

    • Heritability applies to a single trait measured on individuals within a specific population at a given time.

    • It is crucial for making genetic evaluations and predicting responses to selection, aiding breeders in decision-making regarding trait improvement.

  • Inheritance Focus:

    • Heritability illustrates how inheritance operates within populations rather than individuals. This means that the heritability of a trait explains the proportion of total variation attributed to genetic differences among individuals within a population.

Heritability Expression

  • Scale: Heritability is expressed on a scale from 0 to 1 (or 0 to 100%).

    • Ranges indicate the degree of resemblance between offspring and parents for a specific trait.

    • The complement reflects environmental effects.

  • Examples of Heritability:

    • Wool mean fiber diameter: approximately 75% heritability, indicating strong genetic influence.

    • Dairy milk fat and protein concentration: around 70% heritable.

    • Growth rate: moderately heritable at 60%.

    • Reproductive performance: low heritability of 1-2%.

Breeding Implications of Heritability

  • Confidence in Selection:

    • High heritability (above 0.40 or 40%): Individual phenotype serves as a good indicator of genetic worth.

    • Low heritability (below 0.15 or 15%): Less reliable in identifying optimal genetic candidates based on phenotype alone.

  • Trait Improvement Strategies:

    • For high heritability traits, selection is an effective strategy for improvement.

    • For low heritability traits, management practices may be more efficient than selection.

Heritability Estimate Examples (Humans and Cattle)

Human Traits:

  • Height: 0.80 (70%)

  • Body mass index: 0.45-0.80 (67%)

  • Bone mineral density: 0.61 (2)

  • Intelligence: 0.40-0.60

  • Personality Traits: Vary (e.g., Neuroticism: 0.13-0.58)

Cattle Traits:

  • Production traits: Highly heritable (important for dairy industry)

  • Reproductive traits: Low heritability

  • Health traits: Moderate heritability

Calculating Heritability (h²)

Methods:

  1. Broad-Sense Heritability:

    • It examines the proportion of phenotypic variance (VP) due to genotypic variance. (VG)

    • Formula: h2= VG/VP

    • h2= VG/ (VG+ VE)

    • Variance consideration: VP = VG+E,

      where E represents the environmental variance that also contributes to phenotypic expression.

  2. Narrow-Sense Heritability:

    • Simple response to selection estimation.

    • Formula: h^2 = \frac{R}{S}

    • R (response) = progeny mean - parent mean before selection;

    • S (selection differential) = parent mean after selection - parent mean before selection.

      Calculating Example (Narrow-Sense):

  • Given:

    • Progeny mean wean weight: 35kg

    • Parent sheep wean weight (before selection): 32kg

    • Parent mean weight after selection: 36kg

  • Calculate R and S:

    • R = 35 - 32 = 3

    • S = 36 - 32 = 4

  • Therefore: h^2 = \frac{3}{4} = 0.75

    1. Parent-Offspring Regression:

    • Analyze the slope of the regression of offspring traits against parent traits.