Heritability quantifies the proportion of observable variation in a specific trait within a population that can be attributed to genetic differences among individuals. It is fundamental to understanding how traits are passed down through generations and vary across organisms, providing insights into the genetic basis of diverse characteristics.
Understanding Broad Versus Narrow Sense Heritability
Heritability is divided into broad-sense (H²) and narrow-sense (h²) categories. Broad-sense heritability includes all genetic contributions to a trait’s variation: additive genetic effects, dominance, and epistasis. Additive effects are the sum of individual gene contributions. Dominance occurs when one allele masks another at the same locus, while epistasis involves interactions between genes at different loci.
Narrow-sense heritability focuses on the proportion of phenotypic variation due to additive genetic variance. This variance represents the genetic component parents reliably pass to offspring. Unlike dominance and epistatic effects, which are not predictably transmitted, additive genetic effects are consistently inherited. Therefore, narrow-sense heritability is a more accurate predictor of how a trait will respond to selective breeding or evolutionary pressures.
Applications of Narrow Sense Heritability
Narrow-sense heritability is useful in selective breeding programs. In agriculture, breeders rely on these estimates to improve crop yields and livestock productivity. By identifying traits with high narrow-sense heritability, such as increased milk production or disease resistance, breeders can select individuals with desirable characteristics, anticipating these traits will pass to their progeny. This allows for more efficient and predictable genetic improvement.
Beyond agriculture, narrow-sense heritability is a tool in predicting evolutionary change within natural populations. A trait with higher narrow-sense heritability indicates a greater potential to respond to natural selection, allowing populations to adapt more quickly. In human genetics, it helps researchers understand the genetic basis of complex traits and diseases, such as heart disease or schizophrenia. For example, a study might estimate the narrow-sense heritability of schizophrenia to be around 0.23, suggesting that approximately 23% of the variation in disease susceptibility is attributable to additive genetic factors.
Factors Affecting Narrow Sense Heritability Estimates
Narrow-sense heritability is not a fixed value; it varies depending on several factors. Environmental variation within a population influences heritability estimates. For example, if individuals experience a wide range of environmental conditions—such as differences in nutrition, climate, or pathogen exposure—the environmental contribution to trait variation may increase. This can lead to a lower heritability estimate, even if the genetic underpinnings remain the same.
The specific population being studied also affects heritability estimates, as genetic variation differs between populations. A population with more genetic diversity for a trait will likely show a higher heritability estimate. Gene-gene interactions, including dominance and epistasis, also play a role. While narrow-sense heritability accounts for additive genetic variance, these non-additive interactions can still influence the overall genetic variance and the heritability estimate. Heritability is a population-specific measure, reflecting the interplay of genes and environment within a defined group.
What Heritability Does Not Mean
A common misconception is that heritability applies to an individual’s traits, implying a fixed genetic percentage for a personal characteristic. However, heritability is a population-level statistic, describing the proportion of variation in a trait among individuals within a group due to genetic differences, not how much an individual’s trait is determined by genes. For instance, high heritability for height means genetic differences account for much of the height variation among people in that group, not that a single person’s height is 80% genetic.
Heritability is not a static measure; it can change depending on the specific environment and population. Environmental factors can impact trait expression, even for traits with high heritability. For example, while hair color has high heritability, it can be altered with dye, demonstrating that high heritability does not imply a trait cannot be influenced by environmental interventions. Heritability estimates do not identify specific genes or biological mechanisms; they only quantify the proportion of variation attributable to genetic factors.