A central question in biology is whether observed variation in a particular trait is due to environmental or to biological factors, sometimes popularly expressed as the "nature versus nurture" debate. Heritability is a concept that summarizes how much of the variation in a trait is due to variation in genetic factors. Often, this term is used in reference to the resemblance between parents and their offspring. In this context, high heritability implies a strong resemblance between parents and offspring with regard to a specific trait, while low heritability implies a low level of resemblance.
Quantifying Heritability
Phenotypes that vary between the individuals in a population do so because of both environmental factors and the genes that influence traits, as well as various interactions between genes and environmental factors. Unless they are genetically identical (e.g., monozygotic twins in humans, inbred lines in experimental populations, or clones), the individuals in a population tend to vary in the genotypes they have at the loci affecting particular traits. The combined effect of all loci, including possible allelic interactions within loci (dominance) and between loci (epistasis), is the genotypic value. This value creates genetic variation in a population when it varies between individuals. In fact, heritability is formally defined as the proportion of phenotypic variation (VP) that is due to variation in genetic values (VG).
Genotypes or genotypic values are not passed on from parents to progeny; rather, it is the alleles at the loci that influence the traits that are passed on. Therefore, to predict the average genotypic value of progeny and their predicted average phenotype, investigators need to know the effect of alleles in the population rather than the effect of a genotype. The effect of a particular allele on a trait depends on the allele's frequency in the population and the effect of each genotype that includes the allele. This is sometimes termed the average effect of an allele. The additive genetic value of an individual, called the breeding value, is the sum of the average effects of all the alleles the individual carries (Falconer & Mackay, 1996). According to the principles of Mendelian segregation, one allele from each locus is present in each gamete, and in this way, additive genetic values are passed on from parents to progeny. Indeed, because each offspring receives a different set of alleles from its parents, half of the additive genetic variance in the population occurs within families.
Broad-sense heritability, defined as H2 = VG/VP, captures the proportion of phenotypic variation due to genetic values that may include effects due to dominance and epistasis. On the other hand, narrow-sense heritability, h2 = VA/VP, captures only that proportion of genetic variation that is due to additive genetic values (VA). For definitions and decomposition of components of variation, you can read more about phenotypic variance. Note that often, no distinction is made between broad- and narrow-sense heritability; however, narrow-sense h2 is most important in animal and plant selection programs, because response to artificial (and natural) selection depends on additive genetic variance. Moreover, resemblance between relatives is mostly driven by additive genetic variance (Hill et al., 2008).
Given its definition as a ratio of variance components, the value of heritability always lies between 0 and 1. For instance, for height in humans, narrow-sense heritability is approximately 0.8 (Macgregor et al., 2006). For traits associated with fitness in natural populations, heritability is typically 0.1–0.2 (Visscher et al., 2008).
