Write about the preliminary idea of heritability. Give different methods of estimation and write how these estimates are utilized in formation of breeding/selection plans.

What is Heritability?

Heritability (h²) is a statistical measure that describes the proportion of phenotypic variation (P) in a population that is due to genetic variation (G). It's expressed as a number between 0 and 1 (or 0% and 100%). A high heritability indicates that genetic factors play a significant role in determining the trait's expression, while a low heritability suggests environmental factors are more influential. It’s crucial to understand that heritability is a population-specific characteristic, not a fixed property of a gene.

Phenotypic variation (P) is the observable differences in a trait within a population. This variation can arise from genetic differences (G), environmental differences (E), and the interaction between genes and environment (GxE).

The fundamental equation is: P = G + E + GxE

Methods of Estimating Heritability

Several methods exist for estimating heritability, each with its assumptions and limitations. These can be broadly categorized into mass selection, progeny testing, and more advanced methods like Best Linear Unbiased Prediction (BLUP).

1. Mass Selection

Mass selection is the simplest method and involves selecting individuals based solely on their own phenotype. Heritability is estimated as the phenotypic correlation between an individual’s performance and the average performance of its relatives (often siblings). It’s suitable for traits with moderate to high heritability (h² > 0.2).

2. Progeny Testing

Progeny testing involves evaluating the performance of an individual based on the average performance of its offspring. This method is particularly useful for traits with low heritability (h² < 0.2), as it indirectly assesses the genetic contribution of the parent. However, it is time-consuming and requires a large number of progeny per individual.

3. Sibling Testing

Sibling testing involves evaluating the performance of an individual based on the average performance of its siblings. This is a compromise between mass selection and progeny testing, being quicker than progeny testing but more accurate than mass selection for traits with moderate heritability. It is useful for traits that are difficult or expensive to measure directly on the individual.

4. Best Linear Unbiased Prediction (BLUP)

BLUP is a sophisticated statistical method that utilizes all available data, including individual performance, pedigree information, and environmental factors, to estimate breeding values. BLUP accounts for genetic relationships among individuals and provides more accurate heritability estimates, especially in complex genetic systems. It is widely used in modern livestock breeding programs. BLUP incorporates mixed models to account for both fixed and random effects. The development of BLUP was a significant advancement, particularly for dairy cattle breeding in the 1960s.

Method	Suitable Heritability	Advantages	Disadvantages
Mass Selection	0.2 – 0.5	Simple, quick	Less accurate, ignores relatedness
Progeny Testing	< 0.2	More accurate for low heritability	Time-consuming, requires many progeny
Sibling Testing	0.2 – 0.4	Balance of speed and accuracy	Requires accurate pedigree information
BLUP	All levels	Most accurate, accounts for complex relationships	Requires large datasets, sophisticated software

Utilization of Heritability Estimates in Breeding Plans

Heritability estimates are crucial in formulating breeding plans. Different heritability levels necessitate different selection strategies:

• High Heritability (h² > 0.5): Mass selection is often effective. Emphasis is placed on selecting individuals with superior phenotypes.
• Moderate Heritability (0.2 < h² < 0.5): Sibling testing or BLUP can be employed to incorporate information from relatives.
• Low Heritability (h² < 0.2): Progeny testing or BLUP is essential to capture the genetic contribution. Environmental factors play a significant role, so careful environmental control is also important.

Furthermore, the accuracy of selection is influenced by the selection intensity (the proportion of individuals selected for breeding). Higher selection intensity generally leads to greater genetic gain, but also increases the risk of inbreeding.

The National Dairy Development Board (NDDB) utilizes BLUP extensively in its dairy cattle breeding programs. Accurate heritability estimates for milk production and other traits allow for efficient selection of superior bulls for semen production. This has significantly contributed to increased milk yields in Indian dairy farms. National Livestock Mission This mission, under the Department of Animal Husbandry and Dairying, promotes improved livestock breeds and practices, including genetic improvement programs that rely on heritability estimates for selection. 2014 In poultry, heritability estimates for disease resistance traits (e.g., resistance to avian influenza) are crucial. Breeders use progeny testing and genomic selection (a more advanced form of BLUP that incorporates DNA markers) to identify and propagate birds with superior disease resistance. This reduces reliance on antibiotics and improves overall flock health. Improved disease resistance in poultry flocks, reduced antibiotic use, and increased productivity.