Difference between revisions of "Heritability"
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<math>H^2 = \frac{V_G}{V_T}</math> | <math>H^2 = \frac{V_G}{V_T}</math> | ||
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| + | However, in some cases it is easier and/or more useful to estimate narrow sense heritability (''h''<sup>2</sup>). This is the fraction of variation in a phenotype that is due to additive genetic effects. Genetic effects on phenotypic variation can be broken down into additive effects (''V''<sub>''A''</sub>), effects due to dominance (''V''<sub>''D''</sub>), and the variance due to epistatic interactions (''V''<sub>''I''</sub>). | ||
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| + | <math>V_G = V_A + V_D + V_I</math> | ||
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| + | <math>h^2 = \frac{V_A}{V_T}</math> | ||
(too be continued) | (too be continued) | ||
Revision as of 03:15, 21 October 2017
The concept of heritability recognizes that traits are a product of both genetic and non-genetic (referred to as environmental) effects. Some phenotypes (like cystic fibrosis) are predominantly influenced by alleles at one or a few genes and are most usefully thought of in terms of the discrete effects of classical Mendelian genetics. However, other phenotypes (like stature) fall at the opposite end of the scale and are influenced by many small effects of variation at many genes and well as significant environmental effects and are addressed in a collective approach. (While many phenotypes fall intermediate on this scale and may have a few genes of large effect and many genes of smaller effects along with environmental/non-genetic influences.)
When speaking about heritability of complex traits we often do not know precisely which or how many genes are involved. However, this does not mean that we cannot be precise and quantitative in understand and predicting the genetic influence on the trait. The field of quantitative genetics, of which heritability is a part, is necessarily very statistical in nature.
Many phenotypes vary over a range of measurement. The variance of the phenotype can be estimated. Both genetic variation and environmental variation contribute to phenotype variance. If genetic effects and environmental effects are independent of each other (which is a natural place to start) then the total variance of the phenotype (VT) is the sum of the individual genetic (VG) and environmental (VE) variances.
[math]V_T = V_G + V_E[/math]
Broad sense heritability (H2) is the proportion of variation in a phenotype that is due to genetic variation.
[math]H^2 = \frac{V_G}{V_T}[/math]
However, in some cases it is easier and/or more useful to estimate narrow sense heritability (h2). This is the fraction of variation in a phenotype that is due to additive genetic effects. Genetic effects on phenotypic variation can be broken down into additive effects (VA), effects due to dominance (VD), and the variance due to epistatic interactions (VI).
[math]V_G = V_A + V_D + V_I[/math]
[math]h^2 = \frac{V_A}{V_T}[/math]
(too be continued)
