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Heritability is a term used in biology and population genetics to refer to the distribution of variations in phenotypes in a population that comes from genetic variation. Phenotype is the outward appearance of a particular characteristic in an organism — human height is one example. The appearance of a specific trait can be caused either by environmental or genetic factors. Heritability measures the contribution of genetic factors to differences in phenotype. There are two different statistical measures of heritability: broad and narrow sense.
In any given population of organisms, there is a variation of phenotypes. A certain proportion of these phenotypes are produced by environmental conditions, while another proportion is caused by the organism’s genotype, or its genetic makeup. For example, a person’s height is influenced both by his or her genes and by his or her environment. Good nutrition can produce taller people, but so can having tall parents. Both factors are influential in producing the tall phenotype.
Since both environment and genotype contribute to the phenotypic makeup of a population, a proportion of heritability can be established. If all differences between individuals in a population are caused by genetics, heritability is considered to be 1.0. On the other hand, if environment is the only contributing factor to differences in phenotype, and genetics contribute nothing, then heritability is 0.0. Subtracting the measure of heritability from 1.0 gives the measure of environmentability, which is the proportion of phenotypic variation caused by environmental factors.
The two measures of heritability, broad sense and narrow sense, examine different aspects of genetic variance. Genetic variance is the amount of variation in phenotype due to genetic differences among individuals in the population. It differs from heritability, which is strictly a proportional measure. Additive genetic variance refers only to the phenotypic variance inherited through the presence of one allele — a specific form of one gene. Other types of genetic variance, such as dominance variance, analyze the variation in a population caused by the interactions between alleles.
Narrow sense is the more widely used measure because it focuses on one trait of interest. Biologists use this measure to determine how a population may respond to natural selection — the weeding out of certain individuals and their genotypes due to the greater reproductive success of other individuals. Broad sense heritability incorporates all possible sources of genetic variance, including interactions between genes, in its proportional measure.