Quantitative -Additive, dominance, epistatic variation & Gene action

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Polygenic variation; Genotypic variation- additive, dominance, epistatic ;Nature of gene action


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NAVSARI AGRICULTURAL UNIVERSITY N.M. COLLEGE OF AGRICULTURE ASSIGNMENT ON: Nature of gene action - additive, dominance, epistatic and linkage effects Submitted To, Dr. D. A. CHAUHAN Associate Research Scientist Dept. of Genetics & Plant breeding Submitted By, SANTOSH KUMAR TIWARY M.Sc ( Genetics & Plant breeding) Reg. No. – 04-1292-2012 DEPARTMENT OF GENETICS & PLANT BREEDING N.M. COLLEGE OF AGRICULTURE 2013 1

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Genes are the functional units that control Synthesis of proteins which in turn control expression of traits through various biochemical pathways in organisms. Gene Action refers to the behavior or mode of expression of genes in a genetic populations. Gene Action was first studied by Archibald Edward Garrod (1902) for metabolic disorders in man and subsequently by other in smaller organisms like Drosophila , Neurospora and Bacteria. One Gene One Enzymes Hypothesis- Beadle & Tatum on Gene Action In Neurospora . [I] INTRODUCTION 2

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[II] Polygenic variations Polygenic variations can be partitioned into three categories : P=G+E+GE or P=G+E 1. Phenotypic variation : It is the total variability . It includes both genotypic and environmental variation . The variation is measured in terms of phenotypic variance . 2. Genotypic variation: It is the inherent or genetic variability which is unaltered by the environment . It is most useful variability and exploitable by the breeder. It is measured in terms of genotypic variance . It consist of additive, dominance and epistatic components G=D+H+I 3. Environmental variation: It refers to the non heritable variation and caused by the environment . It is measured in terms of error mean squares . The variation in true breeding lines and in F1 is non heritable and hence VE. 3

[III] Types of gene action governing a polygenic trait : : 

[III] Types of gene action governing a polygenic trait : i) Additive (fixable variation) (d) or D ii) Non-additive (unfixable variation) a) Dominance (h) or H b) Epistatic interaction : 1) additive x additive (i) or I 2) additive x dominance (j) or J 3) dominance x dominance (l) or L 4

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1. Additive variance : Arises of from difference between two homozygotes for a gene i.e. AA and aa. It is generally represented by ‘d’ It is fixable and hence selection for traits governed by such variance is effective. It is due to lack of dominance. It is required for the estimation of narrow sense of heritability Breeding value of individual is measured by additive gene effects. General combining ability of a parent is a measure of additive gene effects. 2. Dominance variance or Intra allelic interaction : It is due to the deviation of heterozygote (Aa) phenotype from the average of phenotypic values of the two homozygotes (AA and aa) it is represented by ‘h’. It is due to incomplete or over-dominance. Dominance variance is not fixable . Selection for traits controlled by dominance variance is not effective. Heterosis breeding may be rewarding. Fisher (1918) partitioned the genotypic variation in a ploygenic trait into its 3 components parts : 5

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3. Epistatic variance or Inter allelic Interaction : It is due to interaction between two or more genes represented by ‘e’. Hayman and Mather partitioned the epistatic component into three types of interactions – viz., additive x additive, additive x dominance Dominance x dominance 1. Additive x additive: inter allelic interaction of both alleles at homo zygous condition. Variance due to this type is called additive x additive type of variance. Fixable . 2. Additive x dominance: It is due to inter allelic interaction of one locus in homo zygous and other in hetero zygous condition. Variation due to this is called as additive x dominance variance. Not fixable . Useful for heterosis breeding. 3. Dominance x dominance variance: It is due to inter allelic interaction of both locus in hetero zygous condition. The variation due to this is called as dominance x dominance variance. Not fixable . Useful for heterosis breeding . 7

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Mather (1949) partitioned the phenotypic variance in to three types : 1. Heritable and fixable (additive variance) 2. Heritable and non fixable (dominance and epistatic ) 3. Non heritable and non fixable (environmental) Singh (1990) partitioned total phenotypic variance as : 1. Fixable (additive, additive x additive) 2. Non fixable (dominance, additive x dominance and dominance x dominance and environment) VP= VG +VE = VD+VH+ VI +VE = VD+VH + VAA+VAD+VDD +VE where , VP=phenotypic variance VE=environmental variance VG=genotypic variance 8

Main Features of Gene Action: 

Main Features of Gene Action Gene action is measured in terms of - components of genetic variance or combining ability variance and effects . Depending upon the genetic variance, gene action is of three types , viz. additive gene action, dominance gene action and epistatic gene action. Dominance and epistatic gene actions jointly are referred to as non-additive gene action. Gene action can be studied with the help of various biometrical techniques such as di allel analysis , partial diallel cross, tri allel analysis , quadri allel analysis, line x tester analysis, generation mean analysis , biparental cross and triple test cross analysis. Gene action is affected by various factors . 9

[IV] Factors Affecting Gene Action : 

[IV] Factors Affecting Gene Action Type of Genetic Material Mode of Pollination Mode of Inheritance Existence of Linkage Sample Size Sampling Method Method of Calculation 10

1) Type of Genetic Material: 

1) Type of Genetic Material Gene action in different types of genetic materials Genetic material Types of gene action (a) Self-pollinated species Pure line variety Additive but no genetic variation Mass selected variety Additive and additive epistasis Multilines Additive and additive epistasis Varietal blends Additive and additive epistasis b) Cross-pollinated species Composite variety Additive, dominance and epistasis Synthetic variety Additive, dominance and epistasis Random mating population Additive, dominance and epistasis c) Both self and cross-pollinated species F1 Hybrid Non-additive and no genetic Variation F1 population Additive, dominance and epistasis 11

2) Mode of Pollination: 

2) Mode of Pollination The gene action is greatly influenced by the mode of pollination of a plant species. The additive gene action is associated with homozygosity and, therefore, it is expected to be maximum in self-pollinated species . The non-additive gene action is associated with heterozygosity and, therefore, it is expected to be more in cross-pollinated species and minimum in self-pollinated crops. 12

3) Mode of Inheritance: 

3) Mode of Inheritance Some characters are governed by one or few genes . Such characters are known as qualitative characters or oligogenic characters. On the other hand, some characters are controlled by several genes . Such characters are referred to as quantitative or polygenic characters. Thus inheritance is of two types , viz . oligogenic and polygenic . Cont…. 13

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Polygenic characters are governed by both additive and non-additive types of gene actions, though the additive gene action is predominant in the expression of such characters. On the other hand, oligogenic traits are primarily governed by non-additive types of gene action (dominance and epistasis). In case of oligogenic traits, epistatic variance is of wide spread occurrence, but comparable evidence for polygenic traits is meagre (Frey, 1966). 14

4) Effect of Linkage : 

4) Effect of Linkage The existence of linkage also affects the gene action. Linkage influences gene action by causing an upward or downward bias in the estimates of additive and dominance genetic variances. There are two phases of linkage, viz. coupling and repulsion. A) In case of coupling phase, there is linkage either between dominant genes (AB) or between recessive genes (ab). B) The repulsion phase refers to linkage between dominant and recessive genes (Ab/aB). Linkage disequilibrium can be reduced by random mating of population. In other words, linkage can be broken by repeated intermating of randomly selected plants in segregating populations. Cont…. 15

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Type of linkage Upward bias in Downward bias in Coupling phase (AB/ ab ) Repulsion phase ( Ab / aB ) Additive variance Dominance variance Dominance variance - Additive variance Effect of linkage on Genetic Variances 16

5) Sample Size : 

5) Sample Size Estimates of genetic variance are influenced by the sample size on which the computation is based. Sample size should be adequate to obtain consistent and meaningful results. Small sample may not provide estimates of sufficient reliability If the estimates are based on the entire population, it will give the true genetic variance of that population, but evaluation of entire population is not practically possible . Large sample size will give estimates of genetic variance 17

6) Sampling Method: 

6) Sampling Method Two main sampling methods- Random and Baised sampling a) The random sampling method generally provides true estimates of genetic variance and hence of gene action . b) The biased sampling on the other hand, will not give representative, estimates of genetic variances and thereby gene action. 18

7) Method of Calculation : 

7) Method of Calculation Several biometrical techniques are used for the estimation of genetic variance. The estimates of genetic variance obtained by various methods will vary to some extent. Moreover, use of some mating designs is based on certain genetical assumptions to obtain valid estimates of genetic variance. Failure to meet one or more of these assumptions may result in biased estimates of genetic components of variance. 19

[V] Implication of Gene action in Plant Breeding : 

[V] Implication of Gene action in Plant Breeding Knowledge of gene action is useful to a plant breeder in three principal ways, 1) In the selection of parents for hybridization, 2) In the choice of breeding procedures for the genetic improvement of various quantitative characters, 3) In the estimation of some other genetic parameters . 20

1) Selection of Parents: 

1) Selection of Parents Good general combining parents can be identified by combining ability analysis. In self pollinated species , good general combining parents can be used in the hybridization programme for obtaining superior sergeants in the segregating generations. In cross pollinated species, good general combining parents can be used for the development of synthetic and composite varieties. 21

2) Choice of Breeding Procedure: 

2) Choice of Breeding Procedure The inheritance of yield and most of the yield contributing characters is polygenic in nature and displays continuous variation. The choice of appropriate breeding procedure depends on the type of gene action involved in the expression of these characters in a genetic population . Additive genetic variance is a pre-requisite for gentic gain under selection , because this is the only genetic variance which responds to selection. 22

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Type of gene action Breeding procedure to be followed 1. Self pollinated crops: Additive Pure line selection , mass selection , progeny selection Non additive Heterosis breeding 2. Cross pollinated crops Additive Synthetic, Composite breeding, Recurrent selection for gca . Non-additive Heterosis breeding and Recurrent selection for sca Both additive and non additive Reciprocal recurrent selection. Breeding procedure & gene action : 23

3) Estimation of other Genetic Parameters : 

3) Estimation of other Genetic Parameters Genetic variances which are the relative measures of gene action are used for working out various genetic parameters . For example . , Additive genetic variance is required for the estimation of heritability in narrow sense and response to selection is directly proportional to narrow sense heritability. The additive and dominance variances are also required for the estimation of degree of dominance and various genetic ratios. Cont…. 24

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In intermating populations, additive genetic variance is never exhausted due to self conversion of non-additive genetic variance into additive one. This type of conversion takes place due to fixation of heterozygote’s into homozygote's. In self -pollinated species, additive genetic variance is in abundance in segregating generations and mixtures of several different pure lines. It is also present in adapted populations of outbreeders. Cont…. 25

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Thus, additive genetic variance is of universal occurrence in plant breeding populations. Non-additive variance also exists, but is generally , smaller in magnitude than additive one. In natural plant populations , additive genetic variance is predominant , which is closely followed by dominance variance . Epistatic variance is the lowest in magnitude. 26

[VI] References: 

[VI] References Falconer D.S and Mackay J. 1998. Introduction to Quantitative Genetics. Longman. Naryanan S S and Singh P. 2007. Biometrical Techniques in Plant Breeding. Kalyani. 27