Component of genetic variation

Additive , dominance,epistatic variance etc.

1. Sub.:- Fundamentals of Plant Breeding Course No. :- APB- 5211 Credit hours:- 3(2+1) Lec . Topic :- Components of Genetic Variation Presented by:- Lt. Roshan Parihar , Asstt . Professor Deptt . of Genetics & Plant Breeding Indira Gandhi Krishi Vishwavidyalaya Raipur, C.G. BTC College of Agriculture & Research Station , Sarkanda , Bilaspur ,(CG)-495001

2. Components of Genetic Variation ( अनुवांशिक भिन्नता के घटक )

3. Components of Genetic Variation Variation:- Any observable variability found between two living organism. Types of Variation Phenotypic Variation. The total variability which is observable. Genotypic Variation. The inherent or genetic variation which remains unaltered by environmental changes. Environmental Variation. The non-heritable variation which is entirely due to environmental effects and varies under different environmental conditions. Causes of Variation by genes:- Monogenic Variation: - One or two Distinct dominant genes leads to bring change in trait expression. Eg . petal colour of flowers. Polygenic variation:- When one characteristic is controlled by two or more genes. Often the genes are large in quantity but small in effect. Examples of human polygenic inheritance are height, skin color, eye color and weight. The polygenic variation present in a plant population is of three types, viz., phenotypic, genotypic and environmental.

4. Phenotypic variability अवलोकनीय भिन्नता is observable. It includes both genotypic and environmental variation and, therefore, is also called total variation. It changes under different environmental conditions and is measured in-terms of phenotypic vari­ance. Genotypic variation अनुवांशिक भिन्नता : refers to genetic or inherent variability which remains unaltered by environmental conditions. This type of variability is more useful to a plant breeder for exploitation in selection or hybridization. It is measured in terms of genotypic variance and consists of additive dominance and epistatic components (see later). Environmental variation वातावरणीय भिन्नता is entirely due to environmental effects and varies under different environmental conditions. This uncontrolled variation is measured in terms of error mean variance. The variation in purelines and their F is non-heritable (environmental).

5. Fisher (1918), Additive, Dominance Epistatic Av. Effects on genes. Dev. Fr Mean value, intra-allelic Dev. Fr Mean value inter-allelic 3 types(AA,AD,DD) Wright (1935) Additive variance Non-additive variance. Same as given by fisher Includes Dominant & Epistatic variances Mather (1949). Herit­able fixable Heritable non-fixable Includes additive var. & AA component of Epistasis . Includes dominant var. & AD & DD component of Epistasis Genetic Variance . ( अनुवांशिक प्रसरण ) Heritable portion of total or phenotypic variance. It is of three types, viz. additive, dominance and epistatic Variances Table 1. Classification of genetic variance according to various Authors.

6. 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 : ( योगशील प्रसरण ) It is the component arising from difference between the two homozygotes for a gene, Eg . AA and aa . 2) Dominance Component : , प्रभावी प्रसरण 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 : प्रबल प्रसरण It results from an interaction between two or more genes. Later Hayman and Mather classified the epistatic components into three types interaction viz. Additive X Additive, A dditive X Dominance, Dominance X dominance. Role of Genetic Variance:

7. Additive variance (A):- It refers to that portion of genetic variance which results due to average effects of genes on all segregating loci. Thus it is the component which arises from differences between two homozygotes of a gene, i.e., AA and aa . Main features of additive genetic variance are given below: It is a measure of additive gene action. show lack of dominance , i.e., intermediate expression. It is associated with homozygosity ,max. in self-pollinating crops and minimum in cross. It is fixable and therefore, selection for traits governed by such variance is very effective . It is required for estimation of heritability in narrow sense and response to selection is directly proportional to narrow sense heritability.

8. Breeding value ( प्रजनन मूल्य ) of an individual is measured directly by the additive gene effects. The general combining ability (GCA) effect of a parent is a measure of additive gene effects. It gets depleted proportionate to the improvement made by selection. In natural plant breeding populations, additive variance is the predominant one closely followed by dominance variance. Additive gene action is the chief cause of resemblances between relatives and progeny. Transgressive segregants उत्क्रामी विसंयोजन is the result of additive gene action, which fall outside the range of parents, are obtained due to fixation of dominant and recessive genes in separate individuals, Such segregation occurs when the parents are intermediate to the extreme values of the, Segregating population.

9. Dominance Variance (D):- It refers to the deviation from the additive scheme of gene action resulting from intra-allelic interaction, i.e., interaction between alleles of the same gene or same locus. It is due to the deviation of heterozgote ( Aa ) from the average of two homozygotes (AA and aa ). Main features of dominance variance are briefly discussed below: It is a measure of dominance gene action . Such genes show incomplete dominance, compelete dominance or overdominance . In other words, the heterozygote ( Aa ) does not represent mean value of two parents for a particular character. The parent to which the heterozygote approaches more closely for a particular trait is known as dominant parent and the other as recessive parent.

10. Dominance variance is associated with heterozygosity and, therefore, it is expected to be maximum in cross-pollinating crops and minimum in self-pollinating species. Dominance variance is not fixable and, therefore, selection for traits controlled by such variance is not effective. Dominance variance is the chief cause of heterosis or hybrid vigour . Specific combining ability variance(SCA) is the measure of dominance variance in diallel , partial diallel and line x tester cross analysis. Dominance variance gets depleted through selfing or inbreeding. In natural breeding.populations , dominance variance is always lesser than additive variance.

11. Epistatic variance (I) : It refers to the deviation from additive scheme as a consequence of inter-allelic interaction, i.e., interaction between alleles of two or more different genes or loci. Main features of epistatic variance are given below: Epistatic variance is a measure of epistatic gene action. Epistatic variance is of three types, viz. additive x additive, additive x dominance and dominance x dominance as defined below : Additive x additive.(A X A) It refers to interaction between two or more loci each exhibiting lack of dominance individually. It is denoted as A x A and is fixable. Additive x dominance.(A X D) It refers to interaction between two or more loci, one exhibiting lack of dominance and the other dominance individually. It is denoted as A x D and is non fixable.

12. Dominance x dominance.(D X D):- It refers to interaction of two or more loci, each exhibiting dominance individually. It is represented as D x D and is non-fixable. First type of epistasis is fixable and, therefore, selection is effective for traits governed by such variance, The last two of epistatic variances are unfixable and, therefore, heterosis breeding may be rewarding for traits exhibiting such variance. In case of generation mean analysis, the epistatic gene interactions are classified on the basis of sign of (h) and(l) into two types. When (h) and (l) have the same sign , it is called complementary type and when (h) and (I) have opposite sign , it is termed as duplicate type of epistasis . In natural plant breeding populations, epistatic variance has the lowest magnitude .

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