Narrow Sense Heritability Vs Broad Sense Heritability

Understanding how traits are passed down through generations is a cornerstone of genetics, and breaking down narrow sense heritability versus broad sense heritability is crucial for anyone delving into this field. These concepts help us quantify the degree to which genetic factors influence observable traits, but they do so with different scopes and applications.

Broad Sense Heritability: The Blueprint

Broad sense heritability (H²) measures the proportion of phenotypic variation (observable differences in traits) in a population that is attributable to total genetic variance. It encompasses all genetic effects, including:

• Additive genetic variance (VA): This refers to the variation due to the additive effects of genes. In other words, it's the sum of the individual effects of each allele on the phenotype.
• Dominance genetic variance (VD): This represents the variation due to interactions between alleles at the same gene locus. For example, if one allele masks the expression of another, it contributes to dominance variance.
• Epistatic genetic variance (VI): This captures the variation due to interactions between genes at different loci. It reflects how the effect of one gene can depend on the presence of other genes.

The formula for broad sense heritability is:

H² = VG / VP

Where:

• H² = Broad sense heritability
• VG = Genetic variance (VA + VD + VI)
• VP = Phenotypic variance

Phenotypic variance (VP) is further broken down into:

VP = VG + VE + VGE

Where:

• VE = Environmental variance (variation due to environmental factors)
• VGE = Genetic-environmental interaction variance (variation due to the interaction between genes and the environment).

What Does Broad Sense Heritability Tell Us?

Broad sense heritability gives us an upper limit on the degree to which genetic factors contribute to phenotypic variation in a population. It tells us how much of the observed differences are potentially due to genetic differences. A high H² indicates that a large proportion of the phenotypic variance is due to genetic differences, while a low H² suggests that environmental factors play a more significant role.

Example:

Imagine a study on plant height. If H² for plant height is high (e.g., 0.9), it suggests that most of the variation in height among the plants in the population is due to genetic differences. Conversely, a low H² (e.g., 0.2) would indicate that environmental factors like soil quality, sunlight, and water availability are primarily responsible for the differences in height.

Limitations of Broad Sense Heritability:

• Overestimation: Broad sense heritability can overestimate the actual contribution of genetic factors, especially when dominance and epistatic interactions are significant. These non-additive genetic effects are not reliably passed down from parents to offspring.
• Population-Specific: H² is specific to the population and environment in which it is measured. Changing either the population or the environment can significantly alter the H² value.
• No Prediction of Response to Selection: Broad sense heritability does not predict how a population will respond to selection. Selection acts primarily on the additive genetic variance (VA), not the total genetic variance (VG).

Narrow Sense Heritability: The Predictor of Response

Narrow sense heritability (h²) focuses specifically on the proportion of phenotypic variance that is due to additive genetic variance (VA). This is the key difference between narrow and broad sense heritability. Narrow sense heritability is the most important parameter in selective breeding and evolutionary biology because it predicts the response to selection.

The formula for narrow sense heritability is:

h² = VA / VP

Where:

• h² = Narrow sense heritability
• VA = Additive genetic variance
• VP = Phenotypic variance

Why Additive Genetic Variance Matters:

Additive genetic variance (VA) is the component of genetic variance that is directly transmitted from parents to offspring. It reflects the degree to which the phenotype of an individual is determined by the sum of the individual effects of its genes. Dominance and epistatic effects, while contributing to the overall genetic variance, are not reliably passed down because they depend on specific combinations of alleles and genes.

What Does Narrow Sense Heritability Tell Us?

Narrow sense heritability predicts the extent to which a population will respond to selection. A high h² indicates that selection will be effective in changing the mean phenotype of the population in the next generation. A low h² suggests that selection will be less effective because much of the phenotypic variation is due to environmental factors or non-additive genetic effects.

Example:

Consider a farmer who wants to increase milk production in their dairy cows. The narrow sense heritability (h²) for milk production is 0.3. This means that 30% of the phenotypic variation in milk production is due to additive genetic effects. If the farmer selects the cows with the highest milk production for breeding, the offspring of these cows are likely to produce, on average, more milk than the previous generation. The higher the h², the greater the expected response to selection.

Predicting Response to Selection:

The response to selection (R) can be predicted using the following equation:

R = h² * S

Where:

• R = Response to selection (the change in the mean phenotype from one generation to the next)
• h² = Narrow sense heritability
• S = Selection differential (the difference between the mean phenotype of the selected parents and the mean phenotype of the entire population)

Example:

Suppose the average milk production in a herd of cows is 10,000 liters per year. The farmer selects cows that produce an average of 12,000 liters per year for breeding (S = 2,000 liters). If h² for milk production is 0.3, the predicted response to selection is:

R = 0.3 * 2,000 = 600 liters

This means that the offspring of the selected cows are expected to produce, on average, 10,600 liters of milk per year (10,000 + 600).

Methods for Estimating Narrow Sense Heritability:

Several methods are used to estimate narrow sense heritability:

• Parent-Offspring Regression: This method involves plotting the average phenotype of offspring against the average phenotype of their parents. The slope of the regression line provides an estimate of h².
• Realized Heritability: This is calculated based on the observed response to selection. It is the ratio of the response to selection (R) to the selection differential (S).
• Twin Studies: By comparing the phenotypic similarity of monozygotic (identical) and dizygotic (fraternal) twins, it is possible to estimate the relative contributions of genetic and environmental factors to phenotypic variance. This is more commonly used to estimate broad sense heritability, but can be adapted to estimate narrow sense.
• Animal Models: These are statistical models that use pedigree information and phenotypic data to estimate variance components, including additive genetic variance.

Limitations of Narrow Sense Heritability:

• Still Population-Specific: Like broad sense heritability, h² is specific to the population and environment in which it is measured.
• Assumes Additivity: The equation assumes that gene effects are primarily additive. In reality, dominance and epistasis can influence the response to selection, although their effects are less predictable.
• Environmental Effects: Environmental factors can still play a significant role in determining the phenotype, even when h² is high.

Key Differences Summarized

To solidify understanding, here’s a table summarizing the core distinctions:

Examples to Illustrate the Concepts

Let’s walk through a few examples to showcase how broad and narrow sense heritability play out in different scenarios:

Example 1: Disease Resistance in Plants

A researcher is studying disease resistance in a population of wheat plants. They find that the broad sense heritability (H²) for resistance to a particular fungal disease is 0.8. This indicates that a large proportion of the variation in disease resistance among the plants is due to genetic factors. However, the narrow sense heritability (h²) is only 0.3. This suggests that only a small proportion of the genetic variance is additive, and that dominance and epistatic interactions may be important.

If the researcher selects the most resistant plants for breeding, the response to selection will be limited because the additive genetic variance is low. The offspring of the selected plants may not be as resistant as expected because the non-additive genetic effects are not reliably transmitted.

Example 2: Body Weight in Livestock

A farmer is breeding pigs for increased body weight. The narrow sense heritability (h²) for body weight is 0.6. This indicates that a significant proportion of the variation in body weight is due to additive genetic effects. If the farmer selects the heaviest pigs for breeding, the response to selection is likely to be substantial. The offspring of the selected pigs are expected to be, on average, heavier than the previous generation.

The farmer can use the equation R = h² * S to predict the expected response to selection. If the average body weight in the population is 100 kg and the farmer selects pigs that weigh an average of 120 kg for breeding (S = 20 kg), the predicted response to selection is:

R = 0.6 * 20 = 12 kg

This means that the offspring of the selected pigs are expected to weigh, on average, 112 kg (100 + 12).

Example 3: Human Height

Height in humans is a classic example often used to illustrate heritability. Studies have shown that the broad sense heritability (H²) for height is very high, often around 0.8 or 0.9. This suggests a strong genetic component to height. The narrow sense heritability (h²) is also substantial, typically around 0.7 or 0.8. This indicates that much of the genetic variance is additive, meaning that height is strongly influenced by the cumulative effects of many genes.

While genetics plays a significant role, environmental factors such as nutrition also contribute to height. Even with a high narrow sense heritability, individuals may not reach their full genetic potential for height if they experience malnutrition during childhood.

Example 4: Intelligence (IQ)

The heritability of intelligence, often measured by IQ scores, is a complex and controversial topic. Estimates of broad sense heritability (H²) for IQ vary, but are often in the range of 0.5 to 0.8. This suggests that genetic factors contribute substantially to individual differences in IQ. However, the narrow sense heritability (h²) for IQ is more difficult to estimate and is subject to considerable debate.

Environmental factors, such as education, socioeconomic status, and access to resources, also play a crucial role in shaping intelligence. The relative importance of genetic and environmental factors may also change over the lifespan.

Factors Affecting Heritability Estimates

It’s important to remember that heritability estimates are not fixed values. Several factors can influence the estimates of both broad and narrow sense heritability:

• Population Structure: The genetic makeup of the population being studied can significantly affect heritability estimates. Different populations may have different allele frequencies and different patterns of genetic variation.
• Environmental Variation: The amount of environmental variation in the study population can also influence heritability estimates. In environments with high levels of variation, heritability estimates may be lower because environmental factors obscure the genetic effects.
• Measurement Error: Inaccurate measurements of the phenotype can also affect heritability estimates. The more accurate the measurements, the more reliable the heritability estimates.
• Gene-Environment Interactions: Complex interactions between genes and the environment can make it difficult to accurately estimate heritability.

Practical Applications

Understanding narrow and broad sense heritability has wide-ranging practical applications across various fields:

• Agriculture: Plant and animal breeders use narrow sense heritability to design selective breeding programs aimed at improving desirable traits such as yield, growth rate, disease resistance, and product quality.
• Medicine: Heritability estimates can help identify the genetic basis of diseases and inform strategies for prevention and treatment. Understanding the heritability of risk factors for disease can also help identify individuals who are at higher risk.
• Evolutionary Biology: Narrow sense heritability is a key parameter in evolutionary models that predict how populations will respond to natural selection. It helps us understand how traits evolve over time.
• Conservation Biology: Heritability estimates can be used to assess the genetic diversity of endangered species and to design breeding programs that maximize genetic variation and minimize inbreeding.

Conclusion

While both narrow and broad sense heritability provide valuable insights into the role of genetics in shaping phenotypic variation, it is crucial to understand their distinct meanings and limitations. Broad sense heritability (H²) tells us the potential for genetic influence, while narrow sense heritability (h²) predicts the response to selection. By focusing on additive genetic variance, narrow sense heritability provides a more accurate and practical measure for predicting how populations will respond to selection and for designing effective breeding programs. Recognizing the context, limitations, and appropriate applications of each measure is essential for sound scientific interpretation and decision-making. The interplay between genetics and environment is complex, and heritability estimates are just one piece of the puzzle in understanding the inheritance of traits.