Back to 2. Genes and Behavior

Side Box 2.5. Heritability

A central aspect of Darwin's theory of evolution includes a key statement regarding the action of natural selection on traits that are heritably transmitted between parents and offspring.

Darwin's cousin, Sir Francis Galton coined the term for regression based on the observation that the regression line which predicts offspring phenotype from parent's phenotype always has a slope less than one. The slope of the parent-offspring regression line is also known as heritability. The heritability for any phenotypic trait describes the proportion of the offspring's phenotype that we can predict from a knowledge of the phenotype of both parents. The prediction is not exact and includes some error about the regression line because offspring phenotype includes environmental effects in addition to polygenic factors inherited from parents. Finally, another formulation of heritability is derived from Equation 2.1 (P = G + E):

heritability = h2 = G / P = G / (G + E).

Heritability is the proportion of phenotypic variation (P = G + E) that is due to genetic causes (G). Because phenotypic variation is larger than genetic variation, heritability should have a slope less than one. However, it is possible for measured heritability to be confounded with environment. For example, if parents and offspring share a common environmental factor which makes them more similar by upbringing than genes alone heritability could be inflated.

In principle, a regression based upon any related individuals can be used to estimate heritability. Sibs share the additive effects of alleles, much like parents resemble offspring because of the additive effect of alleles. We could use a correlation between sibs to predict h2, however, sibs have an even greater tendency to share common environmental factors owing to their common rearing environment, and h2 derived from sibs is likely to be inflated owing to shared environment. Sibs not only share additive effects of alleles, a common environment, but they also share another component of variation referred to as dominance variation that makes them resemble each other more so than they do their own parents.