Components of Genetic Variance:
In crop improvements programme, only the genetic components of variation are important because only this component is transmitted to the next generation. According to Fisher in 1918, components of genetic variance divided into three components viz. 1) Additive, 2) Dominance 3) Epistatic
1) Additive Components:
It is the component arising from difference between the two homozygotes for a gene, E. g AA and aa.
2) Dominance Component:
It is due to the deviation of heterozygote ( Aa) phenotype from the average of phenotypic value of the two homozygotes (AA and aa). It is also referred as intra allelic interaction.
3) Epistatic or Interaction Components:
It results from an interaction between two or more genes.
Later Hayman and Mather classified the epistatic components into three types interaction viz.
1) Additive X additive
2) Additive X dominance,
3) Dominance X dominance.
The ratio of genetic variance to the total variance i.e phenotypic variance is known as heritability.
The extent of contribution of genotype to the phenotypic variation for a trait in a population is ordinarily expressed as the ratio of genetic variance to the total variance. i.e Phenotypic variance. Thus heritability denotes the proportion of phenotypic variance that is due to genotype. Heritability may be represented as follows:
Heritability H = VG
VG + VE
Where VG, VP and VE are the genotypic phenotypic and environmental component of variance respectively.
Types of Heritability:
There are two types of heritability viz
1) Broad sense heritability and 2) Narrow sense heritability.
1) Broad Sense Heritability:
It is the ratio of genotypic variance VG to the total phenotypic variance ( VP)
h 2 (bs) = VG/VP or VG/VG+VE
Broad sense heritability estimates are valid for homozygous lines, or populations. However, when we are dealing with segregating generation. The genetic variance consists of additive and dominance component. Since in self pollinated crop we develop homozygous lines, the dominance component will not contribute to the phenotype of homozygous lines derived from a population. Consequently in such cases only the additive component of variation is important. Therefore, for segregating generation broad sense heritability is less important but narrow sense heritability is more important because it cannot realize fully in the offspring.
2) Narrow Sense Heritability:
It is the ratio of additive genetic variance VA to the total phenotypic variance VP ( smith, 1952)
h 2 (ns) = VA/VP = VA/VG + VE
Narrow sense heritability is reliable measures, as it is based on breeding value. The magnitude of narrow sense heritability is always less than or equal to broad sense heritability.
Methods of Estimation of Heritability:
Heritability can be estimated by three different methods.
a) From analysis of variance table of a trial consisting of a large number of genotypes.
b) Estimation of VG and VE from the variance of P1, P2, P3, P4 generation of a cross.
c) Parent – offspring regression upon doubling provides estimates of heritability. Thus, H = 2b, where b is the regression of progeny means on parent value. When heritability is estimated from the above three methods is known as broad sense heritability.
Uses of Heritability:
1) It is useful in predicting the effectiveness of selection.
2) It is also helpful for deciding breeding methods to be followed for effective selection.
3) It gives us an idea about the response of various characters to selection pressure.
4) It is useful in predicting the performance under different degree of intensity of selection.
5) It helps for construction of selection index.
6) Estimates of heritability serve as a useful guide to the breeder, to appreciate the proportion of variation that is due to genotypic or additive effects.