Measure of the extent to which phenotypic within a population can be attributed to differences in the alleles carried by different .
To understand heritability, it is first important to understand populations. Populations consist of multiple, similar, but not necessarily identical individuals. For organisms, these individuals all can potentially (given, of course, compatibility).
Almost no matter what, individuals making up populations, even if they are , will tend to display phenotypic differences. These differences arise due to chance events, otherwise differences in the environment with which they interact, or differences in what alleles they carry. Generally this is expressed as phenotype = genotype + environment + interaction between genotype and environment.
Keeping things simple, the phenotype of any individual can be described as a consequence of some combination of their genotype and the impact of their environment. Phenotypic differences among two individuals therefore similarly must be a product of a combination of differences in their genotypes (i.e., differences in what alleles they carry) and differences in terms of the impact of the environment that they have been exposed to.
Of these phenotypic differences, heritability is the contribution of genotypic variation to those differences, that is, differences in alleles between individuals that result in phenotypic differences among individuals.
Note the following video, but perhaps beware the simplistic take on the value of social policy near the end. There is a lot more to living a successful life than a marginally higher IQ, not all of which necessarily also display high heritabilities ☺:
In human populations such differences are usually explored with where displaying by definition zero differences in genotype with each other (such that any phenotypic differences can be attributed solely to environment) and with often raised in similar environments but not starting out otherwise genetically identical.
If identical twins are raised in different environments, e.g., separated at birth, then the extent of similarity remaining among them (e.g., in terms of , , ) would be described as a consequence of heritability.
For example, if the population differences were "10" (e.g., inches of height) and identical twins despite being separated at birth varied only by "2", then the heritability would be considered to be "8" (= 10 – 2). The differences embodied in 8 (or 80%) is attributed to differences in alleles and the differences embodied in 2 (or 20%) is attributed to differences in environment.
But, again, heritability is a population attribute with 80% heritability not implying that 80% of your height, for example, is due to your genotype and 20% to your environment but rather that 80% of the difference among individuals across the population is due to differences in their genotypes and only 20% due to differences in their environments.
In yet other words, a population that experiences 100% identical environments (particularly for the duration of their lives) must, if phenotypic variation among individuals is present, be experiencing 100% heritability. That is, any phenotypic differences among individuals found within this identically treated population must be due to .
It is as though you were on the market for a . Naturally, there will be differences among the that you look at. Some of those differences will be a function of the properties that the car had when it was brand new (, , ) and some of those differences will be a function of what has happened to the different cars since then (e.g., , how well it was taken car of, , exposure to , how hard it tended to be driven, etc.). For these cars the "heritability" would be the extent to which differences among them can be attributed (more or less) to those differences that were present at the point that the car was originally purchased.