page_title: Heritability
section_title: Example
hierarchical_section_title: Heritability / Definition / Example
caption_attribution_description: English: Illustration of relationship between additive effects and dominance deviations. Drawing made by me.
context_page_description: Heritability is a statistic used in the fields of breeding and genetics that estimates the degree of variation in a phenotypic trait in a population that is due to genetic variation between individuals in that population. It measures how much of the variation of a trait can be attributed to variation of genetic factors, as opposed to variation of environmental factors. The concept of heritability can be expressed in the form of the following question: "What is the proportion of the variation in a given trait within a population that is not explained by the environment or random chance?"
  Other causes of measured variation in a trait are characterized as environmental factors, including observational error. In human studies of heritability these are often apportioned into factors from "shared environment" and "non-shared environment" based on whether they tend to result in persons brought up in the same household being more or less similar to persons who were not.
context_section_description: The simplest genetic model involves a single locus with two alleles (b and B) affecting one quantitative phenotype.
  The number of B alleles can vary from 0, 1, or 2. For any genotype, BᵢBⱼ, the expected phenotype can then be written as the sum of the overall mean, a linear effect, and a dominance deviation:
   = Population mean + Additive Effect () + Dominance Deviation ().
  The additive genetic variance at this locus is the weighted average of the squares of the additive effects:
  where 
  There is a similar relationship for variance of dominance deviations:
  where 
  The linear regression of phenotype on genotype is shown in Figure 1.
caption: Figure 1. Relationship of phenotypic values to additive and dominance effects using a completely dominant locus.
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