Heritability | Animal Husbandry & Veterinary Science Optional for UPSC PDF Download

Introduction

Heritability is a crucial concept in genetics, representing the proportion of observed phenotypic variance resulting from genetic differences among individual genotypes. This includes the additive, dominant, and epistatic variances.

Heritability Estimates

• Heritability estimates quantify the proportion of phenotypic variance in a population attributed to heredity. Subtracting this percentage from 100 provides an estimate of the environmental contribution to variance.

Clarification by Lush (1945)

• Lush emphasized that heritability estimates focus on differences between individuals or groups rather than absolute values. When discussing heritability for a trait, it pertains to the portion of differences in a population attributed to heredity.

Illustrative Example

To illustrate, consider a heritability of 60% for backfat thickness in swine. If the average backfat thickness is 1.4 inches, it does not mean 0.7 inches is hereditary and 0.7 inches is environmental. Instead, 60% of the differences in backfat thickness among individuals in the population are due to heredity.

Narrow vs. Broad Sense Heritability

Importance of Distinction

• Understanding the differences between narrow and broad sense heritability is crucial. The narrower definition focuses on additive genetic variance associated with differences in average breeding values.

Additive vs. Dominance and Epistasis

• In the narrow sense, heritability primarily includes additive gene action. The broad sense encompasses variations due to additive gene action, dominance, and epistasis.

Theoretical Range of Heritability

Extremes and Practical Values

• Theoretical heritability ranges from 0 to 1.0, but extremes are rarely encountered. A specific heritability value describes a trait in a particular population at a given time.

Factors Influencing Heritability

• Heritability values can change due to alterations in additive genetic variants or components of variance. Gene frequency near 0.5 is associated with the highest heritability.

Significance of Heritability Estimates

Predicting Progress in Selection

• Heritability estimates provide insights into potential progress in trait selection. 
• For instance, a 60% heritability for the rate of gain in beef cattle indicates 60% hereditary influence, guiding effective selection for desirable traits.

Correlation and Additive Gene Action

• High heritability implies a strong correlation between phenotype and genotype, making selection based on individual phenotype effective. It also signifies the importance of additive gene action for the trait.
• In conclusion, heritability estimates offer valuable information for informed decision-making in selective breeding and trait improvement.

Challenges of Low Heritability in Traits

In certain cases, heritability estimates for traits can be notably low, often ranging around 10 or 15 percent or even lower. Using the example of Listeria resistance in wine, low heritability suggests a weak correlation between genotype and phenotype.

Implications of Low Heritability

• When heritability is low, the correlation between superior individuals for a trait and the superiority of their offspring diminishes. 
• This poses challenges for making significant progress through traditional breeding methods.

Alternative Strategies for Low Heritability Traits

• To navigate the limitations of low heritability, greater attention must be paid to the performance of collateral relatives and progeny. 
• Specialized methods of selection and mating become necessary for substantial improvements in the herd or stock.

Heritability's Role in Genetic Studies

Predictive Power of Heritability

Heritability plays a crucial role in predicting the reliability of phenotypic values as indicators of breeding value. Understanding the degree of correspondence between phenotypic values and breeding values is essential for effective breeding strategies.

Measuring Heritability

Heritability is defined as the ratio of additive genetic variance to phenotypic variance. This measure expresses the proportion of total variance attributable to the average effects of genes, influencing the resemblance between relatives.

Regression of Breeding Value on Phenotypic Value

Interpreting Heritability

• Heritability can be viewed as the regression of breeding value on phenotypic value. 
• This perspective highlights that an individual's estimated breeding value is the product of its phenotypic value and the heritability of the trait.

Reliability of Phenotypic Values

• Heritability serves as an indicator of the reliability of phenotypic values in guiding breeding decisions. 
• It emphasizes the degree of correspondence between phenotypic values and the underlying breeding values.

Population Dynamics and Environmental Factors

Influence on Heritability

• Heritability is not only a characteristic of a specific trait but is also influenced by the population and environmental circumstances. 
• Changes in any component of variance, genetic or environmental, can affect the calculated heritability.

Population Variation and Gene Frequencies

• The value of heritability is subject to population-specific factors, including gene frequencies and environmental conditions. 
• Variations in genetic components may arise due to the historical context of the population.

Factors Influencing Heritability Estimates

Population Size and Fixation

• Small populations maintained over an extended period, leading to appreciable fixation, are anticipated to exhibit lower heritability compared to larger populations. 
• The environmental influence is contingent upon management conditions, with variable conditions reducing heritability and uniform conditions increasing it.

Population-Specific Heritability

• Heritability values provided for a specific trait must be understood as pertaining to a particular population under specific conditions. 
• Large standard errors in heritability estimates are common due to variations in environmental factors.

Estimating Heritability

Degree of Resemblance Between Relatives

• Heritability is estimated by assessing the degree of resemblance between relatives, expressed as the correlation between their traits. 
• This observed correlation is a proportion of what would be expected if the trait were entirely inherited with additive genetic effects.

Reliability of Relative Relationships

• The choice of relatives for estimating heritability depends on circumstances. Closer relationships, such as parent-offspring and the regression of offspring on parents, are generally more reliable. 
• The full-sib correlation, while providing an upper limit, is considered the least reliable.

Methods of Heritability Estimation

Offspring-Parent Relationship

• Data are collected by measuring parents and determining mean values for their offspring. These mean values may be weighted based on the number of offspring in each family.

Sib Analysis

• In this analysis, multiple males (sires) are randomly mated to several females (dams), producing offspring for measurement. 
• Variance components are analyzed, distinguishing differences between progeny of females mated to the same male and differences between individual offspring of the same female.

Intra-Sire Regression of Offspring on Dams

• For populations where each male is mated to several females, the traditional regression of offspring on mid-parent is inappropriate. 
• Instead, the heritability can be estimated from the average regression of offspring on dams, calculated within groups and then pooled in a weighted manner. This method is commonly employed in farm animals.