Heritability is the proportion of the variation in a phenotype in a specified population that can be attributed to the genotype, or inheritance. It measures how different a population is due to genetics.
Broad-sense heritability refers to all possible genetic combinations, including those that do not pass down to the progeny. Narrow-sense heritability refers specifically to the portion of variance that is heritable. Narrow-sense heritability is often used in animal husbandry.
Studies of heritability in animals often involves one sire creating progeny on a large number of females; since this is not practical for humans, studies on people are generally done using groups of identical twins; fraternal twins; and identical twins separated at birth and raised in different environments. Twin studies allow researchers to define which phenotypes are truly defined by genotype, and which by environment. This can help determine the heritability of intelligence as well as things like an individual's likelihood to contract cancer or suffer from heart disease.
Heritability is often misunderstood. A low heritability is something in which there is little variation—number of fingers, for instance – while a high one is something like eye color, where there are many potential phenotypes.
Web Resources On Heritability
Book Resources On Heritability
Entwined Lives: Twins and What They Tell Us About Human Behavior by Nancy L. Segal
The Extended Phenotype: The Long Reach of the Gene by Richard Dawkins