Additive Genetic Variance
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KeywordsInteraction Genetic Variances Genetic Diversity Bean Weight Individual Phenotypic Differences Epistatic Variance
Additive variation is the total effect on a trait stemming from one or more gene loci. Each locus contributes to the trait in a measurable way.
Within a population, there are numerous potential sources of phenotypic variations. Each of these sources signifies a distinctive underlying origin. These sources of phenotypic variations decide whether that trait has an evolutionary potential and holds an ability to respond to natural/artificial selection or whether it can respond to environmental variations. A large number of investigators across the world are engaged in determining the significance of both the genetic and environmental factors in regulating the phenotypic expression of a particular trait. These types of information will help them in predicting the evolutionary dynamics of a whole population with reference to a particular trait (Waldmann 2001; Fisher et al. 2004; Byers 2008; Saastamoinen 2008).
Phenotypic variability (V P ) within a population occurs because of two major components which are additive. These are genetic variance (V G ) and environmental variance (V E ). This relationship is summarized as (Falconer and Mackay 1981; Lynch and Walsh 1998; Byers 2008):
V P = V G + V E
However, recently with the evolution of epigenetics, the equation is modified as:
V P = V G + V E + V GE
where V GE stands for variance associated with the interaction of genetic and environmental factors.
The genetic variance (V G ) can be further subdivided into three types, i.e., (1) additive genetic variance, (2) dominance variance, and (3) epistatic variance.
Additive genetic variance occurs due to genes which show an additive effect on the quantitative trait. This results in deviance from the mean phenotype due to inheritance of a particular allele and its relative effect on phenotype. It measures the magnitude to which individual phenotype differences can be prophesied due to additive effects of allelic substitutions.
Dominance genetic variance, on the other hand, is associated with dominant gene actions which cover the influence of the recessive alleles at the particular locus. Epistatic genetic variance occurs due to statistical interaction among loci, i.e., gene-by-gene interaction. The genetic basis of this variance is epistasis, and it is called the interaction genetic variance (V I ).
Heritability refers to the proportion of phenotypic variance due to additive genetic variance among individuals. It is often measured as a fraction of the total variance for the trait that is genetic.
The equation can be given as:
H = V G /V P
where H stands for heritability.
It should be noted that here H does not define the fraction that is genetically determined but the fraction of the variability that is genetic. It is worth noticing that in the absence of genetic variation (every individual with the same genotype), V G = 0, and hence H = 0. Contrariwise, if there is no environmental variability (everyone is subjected to same environmental effects), then V E = 0 or V G = V P and, therefore, H = 1. These values are the theoretical parameters for the H.However, for a polygenic trait, H value would lie somewhere between 0 and 1.
For example, if two inbred lines of beans are intercrossed, in the F1, the variance in bean weight is measured as 1.5. The F1 is self-crossed; in the F2, the variance in bean weight comes out to be 6.1. If we have to estimate the broad sense heritability of bean weight in the F2 population, it can be calculated as:
As the variance in F1 is all environmental and variance in F2 is environmental and genetic, therefore:
V E = 1.5 , V P = 6.1; therefore, V G = 6.1 – 1.5 = 4.6.
And heritability of bean weight can be calculated as H = V G /V P , i . e . , 4.6/6.1 = 0.754.
- Byers, D. (2008). Components of phenotypic variance. Nature Education, 1(1), 161.Google Scholar
- Falconer, D. S., & Mackay, T. F. C. (1981). Introduction to quantitative genetics. Longman London Google Scholar.Google Scholar
- Lynch, M., & Walsh, B. (1998). Genetics and analysis of quantitative traits (Vol. 1, pp. 535–557). Sunderland: Sinauer.Google Scholar