Quantitative genetics · BEGINNER LESSON
Heritability
Heritability is a population-specific measure of how much of the observed variation in a trait can be attributed to genetic differences under the studied conditions. It is not a fixed property of the trait itself, and it can change with the population and environment.
What it is and what is measured
Heritability measures variation in a trait within a population, not whether a trait is caused by genes in one person or animal. In the sources, it is described as the relative contribution of genetic inheritance to observed variation, often reported on a 0 to 1 scale. A value closer to 1 means that, in the studied population and setting, genetic differences explain a larger share of the differences seen among individuals. This is a statistical summary of variance, meaning the spread of trait values in a group. [0,1,2]
[1] pmc.ncbi.nlm.nih.gov › articles › PMC13067441Genetic evaluation of flowering, maturity time and ... [2] Heritability of Crohn's disease and ulcerative colitis - PMC - NIH [3] Heritability and shared environmental effects of brain diseases in 12,040 extended families
Broad-sense and narrow-sense heritability
The sources distinguish broad-sense heritability, written as H², from narrow-sense heritability, written as h². Broad-sense heritability includes all genetic effects that contribute to variance, while narrow-sense heritability includes only additive genetic variance, meaning effects that add up across alleles. In one family-based method, narrow-sense heritability can be estimated from the slope of midparent-offspring regression, where offspring values are compared with the average of the two parents. [0,1]
[1] pmc.ncbi.nlm.nih.gov › articles › PMC13067441Genetic evaluation of flowering, maturity time and ... [2] Heritability of Crohn's disease and ulcerative colitis - PMC - NIH
How it is estimated
Heritability can be estimated with classical family designs, twin studies, pedigree analysis, or mixed-effects models. In the cited studies, twin work used an ACE model, which partitions variance into additive genetic effects A, shared environment C, and non-shared environment E. Family studies also used between-family and within-family variance components, and some plant studies estimated h² from regression or H² from variance formulas. These methods try to separate genetic variation from environmental variation. [0,1,2]
[1] pmc.ncbi.nlm.nih.gov › articles › PMC13067441Genetic evaluation of flowering, maturity time and ... [2] Heritability of Crohn's disease and ulcerative colitis - PMC - NIH [3] Heritability and shared environmental effects of brain diseases in 12,040 extended families
What influences the estimate
Heritability depends on the specific population and environment studied. The same trait can have different estimates in different settings because the amount of genetic variation, shared environment, and other factors can change. The sources also note that classical methods can overestimate h² if non-additive genetic effects such as dominance or epistasis are present, or if family members share environmental conditions. Selective mating can also bias regression-based estimates. [0,1]
[1] pmc.ncbi.nlm.nih.gov › articles › PMC13067441Genetic evaluation of flowering, maturity time and ... [2] Heritability of Crohn's disease and ulcerative colitis - PMC - NIH
Limitations and interpretation
A high heritability estimate does not mean a trait is genetically fixed or unaffected by the environment. It means only that, in that population and under those conditions, genetic differences account for a large fraction of the observed variance. The sources also warn about wide confidence intervals, missing heritability in genome-wide studies, and possible bias when samples are not fully representative. [0,1,2]
[1] pmc.ncbi.nlm.nih.gov › articles › PMC13067441Genetic evaluation of flowering, maturity time and ... [2] Heritability of Crohn's disease and ulcerative colitis - PMC - NIH [3] Heritability and shared environmental effects of brain diseases in 12,040 extended families
SEE THE IDEA
How heritability is estimated and interpreted
Measure variation in a population, such as disease status or flowering time.
Separate additive genetics, shared environment, and non-shared environment when possible.
Use twin, family, pedigree, or regression methods to estimate variance components.
Report the fraction of observed variance linked to genetic differences in that setting.
Trait variation is partitioned into sources, and the genetic part becomes the heritability estimate for that population and environment.
Heritability is not a trait itself; it is the fraction of observed variation explained by genetic differences in a specific study context. [0,1,2]
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Sources and further reading
This general lesson was generated from public source material. It explains a concept; it does not interpret a private paper or provide medical advice.