Heritability

Heritability

  • Definition: Heritability is a measure used to describe the proportion of phenotypic variation in a population that can be attributed to genetic variation.

Major Concepts of Heritability

  • Components of Phenotype:

    • Phenotype (P): Result of both genetics (G) and environment (E)

    • Equation: P = G + E

    • Factors influencing phenotype:

    • Feeding

    • Housing

    • Health

    • Stress

Types of Genetic Effects

  • Additive Effects: Contributions of individual alleles to phenotype

  • Dominance Deviation: Interaction between alleles at a single locus affecting phenotype

  • Epistasis: Interaction between genes at different loci affecting phenotype

Example Illustration: Height in Dogs

  • Genotypes:

    • HHHHHH = tall (66 cm)

    • hhhhhh = short (60 cm)

  • Each H allele contributes approximately 1 cm to height.

  • Note: Environmental factors such as nutrition and health can influence height.

Heritability Formulae

Broad Sense Heritability (h²)

  • Equation: h^2 = \frac{\sigma^2G}{\sigma^2P}

  • Describes the proportion of phenotypic variance attributable to all genetic variance.

  • Range: 0 to 1

    • 0 = complete environmental influence (no heritability)

    • 1 = complete genetic influence (full heritability)

  • Includes: All genetic factors—additive, dominance, and interaction.

  • Limitations: Not practical for estimating breeding values.

Narrow Sense Heritability (h²)

  • Definition: Measures the degree of relationship between performance (phenotype) and breeding value for a specific trait.

  • Equation: h^2 = \frac{\sigma^2A}{\sigma^2P}

  • Characteristics:

    • Ignores gene-gene interactions.

    • More reliable indicator of breeding value.

    • Example: Dwarfism affects height in dogs (e.g., HHHhhh = moderate height but HHHhhh + aa = dwarf).

  • Range: 0 to 1

    • 0 = complete environmental effect and other interactions

    • 1 = entirely due to additive genetic effects.

Heritability Estimates

  • Method of Estimation: Based on relative resemblance among individuals, compared to non-relatives.

  • Can vary depending on the population and environmental conditions.

Regression Analysis

  • Correlation Coefficient (r): Indicates the strength of the relationship between mid-parent phenotype and offspring phenotype.

  • Regression example:

    • r = 0.85 (strong positive correlation)

    • r = 0.15 (weak positive correlation)

  • Slope Interpretation: The slope represents the heritability (h²).

Calculating Heritability from Relatives

  • Formulas for Different Relative Comparisons:

    • Parent-offspring: h^2 = 2r

    • Midparent-offspring: h^2 = r

    • Half-sibling: h^2 = 4r

    • Full sibling: h^2 = 2r

    • (where r is the correlation coefficient)

Examples of Heritability Estimates

  • High Heritability (h² ≈ 0.55):

    • Pig backfat: 0.55

    • Sheep wool length: 0.55

    • % butterfat in milk: 0.60

    • % bone in beef: 0.55

    • Average daily gain in beef: 0.35

  • Medium Heritability (0.20 ≤ h² < 0.35):

    • Milk yield: 0.25

    • Time in thoroughbreds: 0.25

  • Low Heritability (h² < 0.20):

    • Egg hatchability: 0.05

    • Pig litter size: 0.10

    • Sheep conception rate: 0.05

Heritability Rules of Thumb

  • Implications for Selection and Improvement of Traits:

    • High Heritability (h² ≈ 0.35 - 0.70): Traits like growth traits allow for quicker improvement by selection.

    • Medium Heritability (h² ≈ 0.20 - 0.30): Traits like behavior have moderate potential for improvement.

    • Low Heritability (h² ≈ 0.05 - 0.15): Traits like fertility are slow to improve due to high environmental influences.

Factors Influencing Low Heritability

  • **Characteristics Leading to Low Heritability: **

    • High environmental impact on the trait.

    • Complicated inheritance involving multiple factors and organ systems.

    • Example in fertility where factors include:

    • Estrus (controlled by hormones)

    • Litter size (affected by ovulation rate)

    • Anatomy

    • Parturition and rebreeding.

Heritability in Companion Animals

  • Dogs Behavioral Traits:

    • Pulling: 0.26

    • Pointing: 0.18

    • Aggression towards strangers: 0.63

    • Trainability: 0.73

    • Chasing: 0.62

Weight and Size Traits in Dogs

  • Fresh data on heritability estimates for various dog characteristics:

    • Weight: 0.41

    • Height: 0.65

    • Head width: 0.35

    • Chest depth: 0.54

    • Leg circumference: 0.08

    • Paw width: 0.72

  • Question Raised: Are a pup’s paws a good predictor of its eventual size?

Repeatability (R)

  • Definition: Measures correlation of repeated measurements from a single individual over time, indicating environmental variability within one genotype.

  • Use case: Example of milk yield in a dairy cow recorded across multiple lactation periods.

  • Examples of Repeatability Values:

    • Litter size in sows: 16%

    • Weaned calf weight in beef cattle: 40%

    • Milk yield in dairy cows: 60%