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H 2 is the broad-sense heritability. This reflects all the genetic contributions to a population's phenotypic variance including additive, dominant , and epistatic (multi-genic interactions), as well as maternal and paternal effects , where individuals are directly affected by their parents' phenotype, such as with milk production in mammals.
Broad-sense heritability, H 2 = V G /V P, Involves the proportion of phenotypic variation due to the effects of additive, dominance, and epistatic variance. Narrow-sense heritability, h 2 = V A /V P, refers to the proportion of phenotypic variation that is due to additive genetic values (V A). [6]
Narrow sense Heritability (h 2 or H N) focuses specifically on the ratio of additive variance (V A) to total phenotypic variance (V P), or: h 2 = V A / V P.. In the study of Heritability, Additive genetic effects are of particular interest in the fields of Conservation, and Artificial selection.
The formula is = where is the broad sense heritability, is the (monozygotic, MZ) identical twin correlation, and is the (dizygotic, DZ) fraternal twin correlation. Falconer's formula assumes the equal contribution of environmental factors in MZ pairs and DZ pairs.
GCTA estimates are likewise sometimes misinterpreted as "lower bounds" on the narrow-sense heritability but this is also incorrect: first because GCTA estimates can be biased (including biased upwards) if the model assumptions are violated, and second because, by definition (and when model assumptions are met), GCTA can provide an unbiased ...
Narrow-sense heritability has been used also for predicting generally the results of artificial selection. In the latter case, however, the broadsense heritability may be more appropriate, as the whole attribute is being altered: not just adaptive capacity. Generally, advance from selection is more rapid the higher the heritability.
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Intuitively, SNP heritability increases to the degree that phenotypic similarity is predicted by genetic similarity at measured SNPs, and is expected to be lower than the true narrow-sense heritability to the degree that measured SNPs fail to tag (typically rare) causal variants. [50]