Model Answer
0 min readIntroduction
Heritability, a cornerstone concept in plant breeding, quantifies the proportion of phenotypic variation in a population that is attributable to genetic variation. It is crucial for predicting the response to selection and guiding breeding strategies. The concept gained prominence with the advancements in quantitative genetics pioneered by Ronald Fisher and Sewall Wright. Recent developments in genomic selection are increasingly refining our understanding and application of heritability estimates, allowing for more precise breeding decisions. Understanding heritability helps breeders determine which traits are most amenable to genetic improvement and prioritize their efforts accordingly.
What is Heritability?
Heritability is a statistical measure that estimates the degree to which phenotypic variation (observable characteristics) in a population is due to genetic factors. It ranges from 0 to 1 (or 0% to 100%), where 1 indicates that all phenotypic variation is genetically determined, and 0 indicates that all variation is due to environmental factors. A higher heritability value suggests that selection for a trait will be more effective.
Types of Heritability
Heritability is broadly classified into two main categories: broad-sense heritability (H2) and narrow-sense heritability (h2). Phenotypic heritability (P2) is also crucial for breeders.
Broad-Sense Heritability (H2)
H2 represents the proportion of phenotypic variance attributable to the total genetic variance in a population. It includes all genetic effects, such as additive, dominance, and epistatic (gene interaction) effects.
Formula: H2 = VG / VP, where VG is the genetic variance and VP is the phenotypic variance.
Significance: H2 is useful for understanding the overall genetic potential for improvement but doesn't directly predict the response to individual plant selection. A high H2 indicates a large genetic potential, but the effectiveness of selection depends on the contribution of additive genetic variance.
Narrow-Sense Heritability (h2)
h2 represents the proportion of phenotypic variance attributable to the additive genetic variance. Additive genetic variance is the component of genetic variance that is passed down linearly from parents to offspring, and it's the most predictable and responsive to selection.
Formula: h2 = VA / VP, where VA is the additive genetic variance and VP is the phenotypic variance.
Significance: h2 is the most important type of heritability for breeders because it directly relates to the expected response to selection. Breeders primarily focus on increasing the frequency of favorable additive alleles.
Phenotypic Heritability (P2)
P2 is the proportion of phenotypic variance due to genetic and environmental variance combined. It’s the easiest to measure, but doesn't distinguish between genetic and environmental effects.
Formula: P2 = VP / VP = 1. It's useful for understanding the overall variability in a population but doesn't provide information about the genetic basis of that variation.
| Type of Heritability | Formula | Components Included | Significance for Breeders |
|---|---|---|---|
| Broad-Sense (H2) | VG / VP | Additive, Dominance, Epistatic | Indicates overall genetic potential |
| Narrow-Sense (h2) | VA / VP | Additive | Predicts response to selection |
| Phenotypic (P2) | 1 | Genetic and Environmental | Describes overall variability |
Significance in Crop Improvement
- Predicting Selection Response: Breeders use h2 to estimate the expected gain in a trait per generation of selection.
- Trait Selection: High h2 values indicate that selection for a trait will be more effective. For example, selecting for disease resistance in rice often has a relatively high h2.
- Breeding Method Choice: Knowledge of heritability helps in choosing appropriate breeding methods. For traits with low h2, hybrid breeding or genetic engineering may be considered.
- Genomic Selection: Modern genomic selection techniques utilize molecular markers to predict breeding values, bypassing the need for extensive phenotypic evaluation, particularly for traits with low heritability.
The Indian Council of Agricultural Research (ICAR) actively promotes research on heritability and genetic parameters in various crops to improve breeding efficiency.
Conclusion
In conclusion, heritability is a vital tool for plant breeders, offering insights into the genetic control of traits and guiding breeding strategies. While broad-sense heritability indicates genetic potential, narrow-sense heritability is paramount for predicting response to selection. The advent of genomic selection is revolutionizing crop improvement, allowing breeders to overcome limitations associated with traits exhibiting low heritability and further optimizing breeding efforts for enhanced food security. Accurate estimation and careful interpretation of heritability values remain crucial for effective crop improvement programs.
Answer Length
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