Model Answer
0 min readIntroduction
Heritability is a fundamental concept in quantitative genetics and animal breeding, quantifying the proportion of observed phenotypic variation in a trait within a population that is attributable to genetic variation among individuals. It provides a critical measure for breeders to understand the extent to which offspring resemble their parents for specific traits and to predict the potential for genetic improvement through selection. This statistical estimate, often denoted as h² (narrow-sense) or H² (broad-sense), is crucial for designing effective breeding programs, determining optimal selection strategies, and distinguishing between genetic and environmental influences on complex traits. Understanding its different types, based on the underlying genetic variance components, is key to its practical application.
Understanding Genetic Variance Components
The total phenotypic variance (VP) of a trait in a population can be partitioned into genetic variance (VG) and environmental variance (VE). The genetic variance itself is further subdivided into components that reflect different modes of gene action. Specifically, VG comprises additive genetic variance (VA), dominance genetic variance (VD), and epistatic genetic variance (VI).- Additive Genetic Variance (VA): This component represents the variation due to the average effects of individual alleles. It is the most important component for selection as it describes the portion of genetic variation that can be directly passed from parents to offspring. VA accounts for the cumulative effect of genes, where each gene contributes independently to the phenotype.
- Dominance Genetic Variance (VD): This arises from interactions between alleles at the same locus (intra-allelic interaction). It accounts for the deviation of the heterozygote's phenotype from the average of the two homozygotes. Dominance effects are specific to genotypes and are not directly transmissible in a simple additive manner from parents to offspring, making them generally non-fixable by selection alone in outbreeding populations.
- Epistatic Genetic Variance (VI): Also known as interaction variance, this component results from interactions between alleles at different loci (inter-allelic interaction). Epistatic effects occur when the expression of one gene is modified by one or more other genes. Like dominance variance, epistatic variance is generally non-additive and complex to exploit in breeding programs.
VG = VA + VD + VI
Types of Heritability
Based on these genetic variance components, heritability is primarily classified into two types: Broad-Sense Heritability and Narrow-Sense Heritability.1. Broad-Sense Heritability (H²)
Broad-sense heritability, denoted by H², measures the proportion of the total phenotypic variance that is due to all genetic factors (additive, dominance, and epistatic interactions). It reflects the total extent to which genetic differences among individuals contribute to the phenotypic variation in a trait.Formula: H² = VG / VP = (VA + VD + VI) / (VG + VE)
Broad-sense heritability is useful for understanding the overall genetic control of a trait but has limited predictive power for selective breeding in sexually reproducing populations because only additive genetic effects are consistently passed from parent to offspring. It is more relevant for predicting response to selection in populations of clones or highly inbred lines where the entire genotype is replicated.2. Narrow-Sense Heritability (h²)
Narrow-sense heritability, denoted by h², is arguably the most crucial type of heritability in animal and plant breeding. It quantifies the proportion of the total phenotypic variance that is attributable solely to additive genetic variance (VA). This is the heritable component that can be effectively exploited through selection to improve a trait in the next generation.Formula: h² = VA / VP = VA / (VA + VD + VI + VE)
Narrow-sense heritability is the primary determinant of the expected response to selection. A high narrow-sense heritability indicates that selection for a particular trait will likely result in significant genetic progress in subsequent generations. This is because additive genetic effects are those that "breed true" and are readily transmitted from parents to offspring.Comparison of Broad-Sense and Narrow-Sense Heritability
The table below summarizes the key differences between the two types of heritability:
| Feature | Broad-Sense Heritability (H²) | Narrow-Sense Heritability (h²) |
|---|---|---|
| Genetic Components | Includes all genetic variances: Additive (VA) + Dominance (VD) + Epistatic (VI) | Includes only Additive genetic variance (VA) |
| Formula | H² = VG / VP | h² = VA / VP |
| Application in Breeding | Limited for predicting selection response in outbreeding populations; more relevant for clones or inbred lines. | Highly crucial for predicting response to selection and designing breeding programs in outbreeding populations. |
| Predictive Power for Selection | Lower predictive power for genetic gain in subsequent generations of sexually reproducing organisms. | Higher predictive power for genetic gain, as VA is the only component directly transmitted to offspring. |
| Interpretation | Total proportion of phenotypic variation due to all genetic effects. | Proportion of phenotypic variation due to transmissible genetic effects. |
Practical Implications in Animal Breeding
The distinction between these heritability types is vital in animal breeding. Traits with high narrow-sense heritability (e.g., carcass quality traits like fat and protein percentage, generally >0.40 in dairy cattle) respond well to direct selection based on individual phenotypes. Conversely, traits with low narrow-sense heritability (e.g., reproductive efficiency, fertility, and disease resistance, typically <0.15), often influenced more by dominance, epistasis, and environmental factors, require more complex breeding strategies like progeny testing, crossbreeding, or incorporating genomic information for effective improvement. Production traits like milk yield often show moderate heritability (0.15 to 0.40).Conclusion
In conclusion, heritability, based on the components of genetic variance, provides a robust framework for understanding the genetic basis of quantitative traits. Broad-sense heritability (H²) encompasses all genetic contributions to phenotypic variation, offering an overall picture of genetic influence. In contrast, narrow-sense heritability (h²), by focusing specifically on additive genetic variance, is the most practical measure for predicting the response to selection in animal breeding programs. By judiciously using heritability estimates, particularly narrow-sense heritability, breeders can make informed decisions to optimize selection strategies, accelerate genetic progress, and develop improved animal populations that are more productive, resilient, and efficient.
Answer Length
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