Define Heritability. Discuss any one method used for its estimation. How are the estimates useful in animal breeding ?

Defining Heritability

Heritability (h²) is a statistical measure that represents the proportion of phenotypic variation (P) in a population that can be attributed to genetic variation (G). It's a population-specific measure, meaning it reflects the genetic and environmental conditions of a particular population at a specific time. It is not a measure of the genetic contribution of a single individual. Mathematically, it’s expressed as:

h² = G / P

Where:

• P = Phenotypic variance (total variation observed in a trait)
• G = Genetic variance (variation due to genetic factors)

Genetic variance is further subdivided into:

• Additive genetic variance (A): Effects of individual genes add up.
• Dominance genetic variance (D): Interactions between genes.
• Epistatic genetic variance (I): Interactions between different genes.

Therefore, G = A + D + I. A higher heritability value indicates that genetic factors play a more significant role in determining the trait's expression.

Twin and Adoption Studies for Estimating Heritability

Twin and adoption studies are widely used to estimate heritability. They exploit the differences in genetic relatedness between individuals to disentangle the effects of genes and environment.

Twin Studies

Twin studies compare monozygotic (MZ) twins (identical, sharing 100% of their genes) and dizygotic (DZ) twins (fraternal, sharing approximately 50% of their genes). The underlying assumption is that if MZ twins are more similar for a trait than DZ twins, then genetics are playing a larger role.

The formula for estimating heritability from twin data is:

h² = 2 * [(r_MZ - r_DZ) / (1 - r_DZ)]

Where:

• r_MZ = correlation coefficient for MZ twins
• r_DZ = correlation coefficient for DZ twins

Example: If r_MZ = 0.8 and r_DZ = 0.6, then h² = 2 * [(0.8 - 0.6) / (1 - 0.6)] = 0.4. This suggests that 40% of the phenotypic variation in the trait is due to genetic factors.

Adoption Studies

Adoption studies compare adopted individuals with their biological and adoptive parents. If an adopted individual resembles their biological parent more than their adoptive parent for a particular trait, it suggests a genetic influence. The heritability is estimated by comparing the correlations between adopted individuals and their biological parents versus their adoptive parents.

Limitations of Twin and Adoption Studies:

• Assumption of Equal Environments: Both types of studies assume that MZ twins experience similar environments and adopted individuals experience environments similar to their adoptive families. This assumption may not always hold true.
• Gene-Environment Interaction: These methods may not adequately account for gene-environment interactions, where the effect of genes depends on the environment.
• Population Specificity: Heritability estimates are specific to the population and environment in which the study is conducted.

Usefulness of Heritability Estimates in Animal Breeding

Heritability estimates are indispensable in animal breeding programs for several reasons:

• Selection Response Prediction: Heritability helps predict the expected response to selection. The higher the heritability, the greater the expected genetic improvement in a trait per generation of selection. The response to selection (R) can be calculated as: R = h² * S, where S is the selection differential (the difference between the mean of the selected parents and the mean of the population).
• Breeding Strategy Design: Knowing the heritability of a trait informs the choice of breeding strategies. For highly heritable traits, individual selection (selecting animals based on their own performance) is effective. For traits with low heritability, family selection (selecting based on the performance of relatives) or combined selection may be more appropriate.
• Genetic Improvement Programs: Heritability is used to determine the intensity of selection and the number of animals to be included in breeding programs.
• Identifying Traits for Improvement: Traits with moderate to high heritability are generally prioritized for breeding efforts.

Example: Milk yield in dairy cattle is a moderately heritable trait (around 0.3). Breeders use this information to select cows with high milk production potential and their offspring, aiming to improve overall milk yield in the herd. In contrast, temperament, which is often less heritable, might require a more complex breeding strategy involving family selection and careful management.

Scheme: The National Livestock Mission (NLM) under the Department of Animal Husbandry and Dairying utilizes genetic improvement programs based on heritability estimates to enhance the productivity and quality of livestock.

Case Study: Dairy Cattle Breeding in the Netherlands

The Dutch dairy cattle breeding program is a global leader in genetic improvement. They employ sophisticated genomic selection techniques, relying heavily on accurate heritability estimates for various traits like milk yield, fat, and protein content. The program utilizes a national database of phenotypic and genomic information, allowing for precise selection of animals with superior genetic merit. This has resulted in significant improvements in milk production and quality over decades, demonstrating the power of leveraging heritability estimates in a practical breeding context.