types of gene action and there implication in plant breeding


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Types Of Gene Action And There Implication In Plant Breeding :

Types Of Gene Action And There Implication In Plant Breeding Presented By Mr. Arun Dhanaji Ugale M.Sc.(Agri.) , Genetics And Plant Breeding


GENE ACTION Gene are the functional units that govern the development of various characters of an individual Gene Action refers to the behavior or mode of expression of genes in a genetic populations. Genes control Synthesis of proteins which in turn control expression of various traits of in organisms. INTRODUCTION Cont….

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Gene action is under stood through the study of various biochemical nature of gene and there mode of action in biochemical pathways. Gene Action was first studied by Archibald Edward Garrod (1902) in man and subsequently by other in smaller organisms like Drosophila , Neurospora and Bacteria who was an English Physician .

Gene Action in Man:

Gene Action in Man Garrod studied gene action man an reported for the first time in 1902 that certain human diseases or defects are hereditary. Such metabolic disorders are caused by recessive mutation or biochemical defects. Later on1909 he postulated that inborn errors of metabolism due to enzymes defects and such defects of enzymes are heritable . He wrote a book entitled. ‘‘ Inborn Errors of Metabolism ’’ in which he explained the causes of various metabolic disorders. Some of the diseases caused by metabolic disorder in man includes 1.Alkaptonuria 2.Phenylketonuria 3.Albinism 4.Tyrosinosis 5.Goitrous Cretinism.

Gene Action In Drosophila:

Gene Action In Drosophila Beadle And Ephrussi used Drosophila as experimental material for the study of gene action. Drosophila is very convenient for handling in laboratory studies They made eye transplantation with Drosophila and concluded in 1937 that genes control biochemical reaction which in turn control the expression of eye pigments in Drosophila.

Gene Action In Neurospora :

Gene Action In Neurospora Beadle could not get clear picture of gene action in his studies with Drosophila. Henes he later on switched over his investigation on gene action from Drosophila to pink bread mould ( Neurospora crassa ). Beadle conducted his experiments on Neurospora in collaboration with Tanum . They demonstrated for the first time in 1941 that gene control expression of characters through synthesis of enzymes which are proteins . They develop the concept of One Gene One Enzymes Hypothesis .

Gene Action In Plant Breeding :

Gene Action In Plant Breeding Knowledge of gene action helps in the selection of parents for use in the hybridization programmes and also in the choice of appropriate breeding procedure for the genetics improvement of various quantitative characters. klence insight into the nature of gene action involved in the expression of various quantitative characters is essential to a plant breeder for starting a judicious breeding programme .

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. Cont….

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Gene action can be studied with the help of various biometrical techniques such as diallel analysis, partial diallel cross, triallel analysis, quadriallel analysis, line x tester analysis, generation mean analysis, biparental cross and triple test cross analysis. Gene action is affected by various factors. Cont….

Gene Action and Plant Breeding:

Gene Action and Plant Breeding The science of plant genetics traces back to Mendel’s classic studies with garden peas. Mendel evaluated crosses of pure lines and by scoring phenotypes he deduced the existence of genes and determined their mode of action. Plant breeders identify superior genotypes and develop new cultivars by selecting plants possessing desirable phenotypes derived from genetic recombination. Cont….

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The understanding of gene action is of paramount importance to plant breeders. Alleles with a dominant, additive, or deleterious phenotypic effect affect heritability differently depending on whether they are in homozygous or heterozygous condition. An understanding of how heterozygosity and homozygosity affect gene action and interaction will facilitate decisions about whether the end product in breeding programs should be hybrids or inbred lines. Cont….

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Knowledge of the way genes act and interact will determine which breeding system optimizes gene action more efficiently and will elucidate the role of the breeding systems in the evolution of crop plants. Darwin (1876), ignoring the existence of genes and gene action, discussed the significance of breeding systems in his book: The Effects of Cross and Self-Fertilization in the Vegetable Kingdom. He stated that “the first and most important of the conclusions which may be drawn from observations given in this volume, is that generally cross-fertilization is beneficial and self-fertilization often injurious.” Cont….

Types Of Gene Action:

Types Of Gene Action Allelic Gene Action and Interaction Positive Dominance Semidominance Codominance Negative Dominance Implications in Breeding Nonallelic Gene Action and Interaction Positive Epistasis Semiepistasis Coepistasis Negative Epistasis Implications in Breeding

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Gene Action and Systems of Reproduction Self-Fertilization Cross-Fertilization Asexual Reproduction Genetic Basis of Heterosis Implications in Breeding Conditions Optimizing Single-Plant Heritability A. Absence of Competition Single-Plant Phenotypic Expression and Differentiation. Genetic and Environmental Variances as Components of Phenotypic Variance. Correspondence Between Genotypic and Phenotypic Values. B. Enhanced Gene Fixation C. Multiple-Environment Screening D. Prediction Criteria E. Implications in Breeding Cont….

Factors Affecting Gene Action :

Factors Affecting Gene Action Type of Genetic Material Mode of Pollination Mode of Inheritance Existence of Linkage Sample Size Sampling Method Method of Calculation Cont….

Type of Genetic Material:

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

Mode of Pollination:

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.

Mode of Inheritance:

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.

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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 widespread occurrence, but comparable evidence for polygenic traits is meagre (Frey, 1966).

Existence of Linkage :

Existence 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. In case of coupling phase, there is linkage either between dominant genes (AB) or between recessive genes (ab). The repulsion phase refers to linkage between dominant and recessive genes (Ab/aB). Cont….

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

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High frequency of coupling phase (AB/ab) causes an upward bias in the estimates of additive and dominance variances ( Hallauer and Miranda, 1981 ). An excess of repulsion phase linkage (Ab/aB) leads to upward bias in dominance variance and downward bias in the additive variance . 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. The number of intermating generations required for breaking the linkage depends on the closeness of the linkage ( Hanson, 19.59 ). Cont….

Sample Size :

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

Sampling Method:

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

Method of Calculation :

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.

Implication In Plant Breeding :

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

Selection of Parents:

Selection of Parents Selection of parents for hybridization is an important step in plant breeding. 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 and in cross pollinated species such parents can be used for the development of synthetic and composite varieties.

Choice of Breeding Procedure:

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.

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Additive genetic variance gets depleted proportionate to the improvement made by selection ( Arunachalam , 1989). In other words, genetic improvement through selection is achieved at the expense of additive genetic variance. In pure line selection, additive genetic variance is completely depleted. That is why further improvement through selection is not possible in a pure line population. In pure lines, additive genetic variance is regenerated over a period of time by recombination and mutation. Cont….

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If there is preponderance of additive gene action, reliance should be placed on mass selection and progeny selection in self-pollinated species and synthetic and composite breeding in cross pollinated species. Non-additive gene action is a prerequisite for launching a heterosis breeding programme . If there is preponderance of non-additive gene action, the breeding objective should be towards development of hybrids for commercial purpose. Cont….

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If both additive and non-additive gene actions are of equal magnitude, population improvement programmer should be taken up for the development of superior lines with several desirable genes. In cross-pollinated species, various types of recurrent selections are used depending upon the relative importance of gene action. Recurrent selection for general combining ability is effective with additive gene effects; recurrent selection for sca makes use of non-additive gene effects; and reciprocal recurrent selection utilizes both additive and non-additive gene effects. Cont….

Estimation of other Genetic Parameters :

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….

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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 outbrreders . Cont….

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Thus, additive genetic variance is of universal occurrence in plant breeding populations. Non-additive variance also exists, but is generally ,rnaller 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. Cont….


References Phundan Singh , Elements Of Genetics. B.D. Singh , Essential of Plant Breeding.

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