Heritability, Repeatability and Selection Models | Animal Husbandry & Veterinary Science Optional for UPSC PDF Download

Heritability

• Heritability is the proportion of the observed phenotypic variance that arises from genetic differences among genes and gene combinations within individuals.
• It encompasses the additive genetic, dominance, and epistatic variances in a broad sense.
• Heritability estimates indicate the portion of phenotypic variance in a population attributed to genetics.
• The percentage of heritability subtracted from 100 reveals the environmental contribution to the variance.
• For instance, if the heritability of backfat thickness in swine is 50%, it means that approximately 50% of the differences in backfat thickness among individuals are genetic.
• Heritability can be interpreted narrowly or broadly. The narrow definition focuses on additive genetic variance, while the broad sense includes dominance and epistasis effects.
• Theoretical heritability ranges from 0 to 1.0, with actual values varying based on the population and time.
• Additive genetic variance is closely linked to gene frequency, with higher values when gene frequency is around 0.5.

Definition and Importance: 

• Heritability estimates reveal the degree of genetic influence on traits compared to environmental factors. 
• They guide selection processes by indicating the extent of genetic contribution to a specific trait. 
• For instance, a 60% heritability for the rate of gain in beef cattle suggests that 60% of the variation is due to genetics.

Implications of High Heritability: 

• High heritability suggests a strong correlation between phenotype and genotype. 
• Selection based on phenotype is effective when heritability is high. 
• Additive gene action plays a crucial role, making mating the best individuals beneficial for offspring quality.

Challenges with Low Heritability: 

• Traits like litter size in swine may have heritability as low as 10-15%, indicating a weak genotype-phenotype correlation. 
• Selecting superior individuals for low heritability traits may not yield significantly superior offspring. 
• Strategies focusing on collateral relatives and progeny performance become essential for progress.

Non-Additive Gene Action: 

• In cases of low heritability, non-additive gene actions like overdominance and epistasis play significant roles. 
• Specialized selection and mating methods are necessary for substantial herd or flock improvements.

Repeatability

Relation to Heritability: 

• Repeatability is closely connected to heritability and applies to traits expressed multiple times during an animal's lifetime. 
• It should at least be as significant as heritability since the genes influencing trait expression remain constant.

Examples of Repeatability: 

• Traits like lactation milk yield in dairy cows and litter weight in swine exemplify traits where repeatability is crucial. 
• Repeatability estimates indicate the consistency of trait expression in an individual over time.

Prediction and Calculation: 

• Repeatability can be calculated as the regression of future performance on past performance. 
• It represents the fraction of differences between single records that are likely to recur in future records of the same individuals.

Example: Imagine a dairy farm where the heritability of milk production is 70%. This suggests that 70% of the variation in milk yield is due to genetics. Now, consider a scenario where two cows with similar environmental conditions show different milk production levels. The one with higher genetic potential, based on heritability, is likely to consistently produce more milk over successive lactations. This emphasizes the importance of understanding heritability in making informed breeding decisions for optimal results.

Context:

• The study focused on a group of cows in an setting, where selection was based on record size and environmental circumstances, which were below average.
• Over time, the study incorporated a new sample representing both low and high groups of cows, revealing only the real differences between the groups in later records.
• This process aimed to approximate how future performance relates to present performance, serving as an estimate of repeatability.

Repeatability Estimation:

• The repeatability of differences in fat yields during a single lactation period in the data was determined to be 43.6.
• The averages for daughter lactations from high and low groups of cows were compared. Daughters from high groups averaged 393.4 lb, while those from the low group averaged 379.3 lb.

Sire Contribution:

• Each sire had an equal number of daughters in both the low and high groups, ensuring that sire differences did not influence the 14.1 lb difference between daughters of these two groups of dams.