The term “heritable” in genetics refers to a specific, measurable concept that frequently confuses the public. When a trait is described as heritable, it means that differences in the genes of individuals contribute to the observable differences in that trait across a population. Understanding this term is foundational to accurately discussing the roles of genetics and environment. Heritability is a statistical measure that quantifies the degree of genetic influence on trait variation, not simply the presence of genes.
The Core Concept of Heritability
Heritability is a population-level statistic that estimates the extent to which genetic variation contributes to the total variation of a trait within a specific group of people. This statistic, often represented as \(h^2\), ranges from 0 to 1 (0% to 100%). An estimate near 1.0 suggests that nearly all observed differences are due to genetic differences, while an estimate near 0.0 indicates that environmental differences are primarily responsible for the variation.
For example, the heritability of human height is estimated to be around 0.80, meaning 80% of the variation in height among people is attributable to genetic differences. This measure does not mean that 80% of any single person’s height is determined by their genes.
Scientists distinguish between broad-sense heritability (\(H^2\)) and narrow-sense heritability (\(h^2\)). \(H^2\) considers the total genetic contribution to variation, including all forms of gene interaction, such as dominant and epistatic effects. \(h^2\) focuses only on the additive genetic variance, which represents the effects of genes passed predictably from parent to offspring.
Narrow-sense heritability is particularly useful in fields like selective breeding because it predicts how much a trait will respond to selection. The difference between the two measures helps researchers understand the complexity of the underlying genetic architecture of a trait.
Heritability Versus Genetic Inheritance
It is important to distinguish heritability from the general concept of genetic inheritance. Genetic inheritance refers to the biological process by which genes are physically passed down from parents to their offspring. Traits are inherited through specific mechanisms, such as Mendelian patterns where a single gene determines a distinct characteristic, like blood type.
Heritability, conversely, is a statistical measure of variation in a trait among a population. A trait can be highly inherited—meaning genes are required for its existence—but have low heritability if there is little variation across the population. For example, having two eyes is inherited, but because virtually everyone has two eyes, the heritability of “number of eyes” is near zero, as there is no variation to explain.
Heritability applies primarily to quantitative or continuous traits, which are characteristics measured along a spectrum, such as weight, intelligence, or blood pressure. These traits are influenced by multiple genes acting together, alongside environmental factors. The heritability of a complex trait like the variation in height is the statistical measure of how much genetic differences contribute to who is tall and who is short.
How Scientists Measure Heritability
Scientists primarily use study designs that leverage natural experiments in human populations to estimate heritability.
Twin Studies
The classical twin study design compares the trait similarity of identical (monozygotic) twins to that of fraternal (dizygotic) twins. Identical twins share nearly 100% of their DNA, while fraternal twins share about 50%. Since both types of twins typically share the same family environment, if identical twins are significantly more similar for a trait, the increased similarity is attributed to the greater genetic overlap. Researchers use mathematical models based on this difference to calculate the heritability estimate.
Adoption Studies
Adoption studies provide another powerful tool by separating genetic and environmental influences. These studies compare adopted children to their biological parents (shared genes, different environment) and their adoptive parents (shared environment, different genes). If an adopted child’s trait more closely resembles that of their biological parents, it suggests a greater genetic contribution, allowing scientists to estimate heritability.
Combining data from twin and adoption studies allows researchers to partition the total variation of a trait into components: additive genetic effects, shared environmental factors, and unique environmental factors. These studies use correlation coefficients to describe the strength of the relationship between relatives for a specific trait, which helps calculate the final heritability estimate.
What Heritability Does Not Mean
A high heritability estimate for a trait does not mean that the trait is unchangeable or predetermined. This is a common misconception that confuses a population statistic with an individual fate. A heritability of 0.70 means that 70% of the variation among people is due to genetic differences, not that environmental factors are irrelevant to the trait’s expression.
Environmental changes can still profoundly modify a highly heritable trait. For instance, the high heritability of Phenylketonuria (PKU) does not prevent effective treatment through a strict dietary intervention. Similarly, height is highly heritable, but improvements in childhood nutrition have led to significant increases in average height across populations, demonstrating strong environmental influence.
Heritability estimates are also context-dependent; they are specific to the population studied and the environment at the time of measurement. If the environment becomes more uniform, the observed variation would be more attributable to genetic differences, potentially increasing the heritability estimate. Conversely, if a population moves to an environment with greater social or nutritional variation, the heritability of some traits may decrease.