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  Section: Genetics » Quantitative Inheritance
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Heritability in broad sense and narrow sense

Quantitative Inheritance
Multiple factors
Binomial distribution
Effect of degree of dominance, number of genes and environment
Quantifying variability 
Variance and standard deviation
Partitioning of phenotypic variance
Heritability in broad sense and narrow sense

Heritability in broad sense (H2) is calculated using the following formula :

H2 = Sg2/Sp2

Therefore, H2 simply gives a measure of the proportion of phenotypic variance which is due to genotype. However, this does not tell us what proportion of an individual's phenotype is due to genotype. Moreover, H2 depends on the population in which it is measured and also depends on the set of environments in which the population was grown. For instance in a genotypically homogeneous population (e.g. homozygous parents or F1 hybrids between them), Sg2 = 0 and therefore H2 = 0. It does not mean that genes do not control the phenotype, but no part of the variation can be ascribed to genetic variation. Similarly lack of heterogeneity in environment may lead to a high estimate of H2, but it does not mean that the trait is insensitive to all environments.

Since H2 can not tell us how will a particular trait behave in a particular environment, its utility is limited. In view of this, animal and plant breeders often prefer to estimate heritability in narrow sense (h2), which is based on the knowledge about gene action. For this purpose genetic variance (Sg2) is partitioned into additive genetic variance (Sa2) and dominance genetic variance (Sd2). If A and a are two alleles at a locus controlling a quantitative trait, the Sa2 is associated with the average effect of substitution of A for a in a homozygote (aa →AA) and Sd2is associated with partial dominance of A over a in heterozygotes. Once Sa2is calculated from given data, Sd2can be calculated as follows (The details of methods for computations of Sa2 and Sd2are beyond the scope of this section and the reader is advised to consult a book on Biometrical Genetics for further details) :

Sd2= Sg2- Sa2.

The heritability in narrow sense (h2) is used as a measure of relative proportion of Sa2 as follows :

h2 = Sa2/ Sp2 (as against H2 = Sg2/ Sp2)

The usefulness of this partitioning of genetic variance lies in the prediction about the effect of selective breeding. Greater is the value of h2, better are the chances that the selected individuals will maintain their superiority over the population as a whole. This kind of assurance and confidence is not available when we use H2 (heritability in broad sense). Further, if h2 is high, the selection will bring about an improvement, but if h2 is low it means that Sd2 should be high and heterozygotes will be superior to homozygotes and heterosis breeding should be preferred as done in maize.

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