In this article we will discuss about:- 1. Meaning and Features of Gene Action 2. Factors Affecting Gene Action 3. Role.
Meaning and Features of Gene Action:
Gene action refers to the behaviour or mode of expression of genes in a genetic population. 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 genetic improvement of various quantitative characters. Hence 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 are briefly presented below:
i. Gene action is measured in terms of components of genetic variance or combining ability variance and effects.
ii. 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.
iii. 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.
iv. Gene action is affected by various factors.
Factors Affecting Gene Action:
Since genetic variances are used as measures of gene action, all those factors which affect estimates of genetic variance also affect gene action. Such factors include, type of genetic material, mode of pollination, mode of inheritance, existence of linkage, sample size, sampling method and method of calculation.
These are briefly described below:
1. Type of Genetic Material:
The magnitude of gene action is largely governed by the type of genetic material used for the study. In an F2 or advanced generation of a cross between two pure lines, the genetic variance includes additive, dominance and epistatic components. But in case of homozygous lines, the genetic variance is entirely of additive and additive-epistatic types.
Thus in terms of genetic variance, self and cross pollinated species differ primarily in the relative magnitude of dominance component. In other words in homozygous genotypes, the genetic variance is entirely of additive and additive-epistatic types, while in the segregating populations both additive and non-additive types of gene actions are present (Table 12.2).
In F2, the phenotypic variance has 1/2 D (additive) and 1/4 H (dominance) components. In a random mating population with no epistasis and zero inbreeding, the covariance between a parent and its offspring is 1/2 VA; the covariance among half-sibs is 1/4 VA; and the covariance among full-sibs is 1/2 VA + 1/4 VD. These relationships change with different levels of inbreeding in the population.
The nature of gene action for important agronomic characters in almost all the crops is mainly of additive type. The non-additive type of gene action also exists in nearly all crops and for many important traits, but is generally smaller in magnitude than additive components.
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. Inbreeding increases the amount of additive genetic variance in a population due to increase in homozygosity by way of gene fixation.
On the other hand, outbreeding increases the proportion of non-additive genetic variance by way of creating heterozygosity in a population. Thus gene action changes with the mode of pollination (Fig. 12.1). In cross-pollinated species, selfing or inbreeding leads to conversion of non-additive gene action into additive by way of converting heterozygotes into homozygotes. With single gene, more than ten generations of selfing are required for complete conversion of heterozygotes into homozygotes.
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.
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 meager.
4. 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). High frequency of coupling phase (AB/ab) causes an upward bias in the estimates of additive and dominance variances. An excess of repulsion phase linkage (Ab/ aB) leads to upward bias in dominance variance and downward bias in the additive variance (Table 12.3).
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.
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. However, sample size may vary with the magnitude of genetic variability present in the genetic population under study.
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 nearer to the population mean, while small sample may give biased estimates.
6. Sampling Method:
There are two main sampling methods, viz. random and biased 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. Hence, the material to be evaluated should be a random sample of all possible genotypes present in the population.
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.
Role in Plant Breeding:
Knowledge of gene action is useful to a plant breeder in three principal ways, viz.:
(1) In the selection of parents for hybridization,
(2) In the choice of breeding procedures for the genetic improvement of various quantitative characters, and
(3) In the estimation of some other genetic parameters.
These are briefly described below:
(1) 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 segregates in the segregating generations and in cross pollinated species such parents can be used for the development of syntheic and composite varieties.
(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 (Table 12.4).
Additive genetic variance is a pre-requisite for genetic gain under selection, because this is the only genetic variance which responds to selection. Additive genetic variance gets depleted proportionate to the improvement made by selection.
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.
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. If both additive and non-additive gene actions are of equal magnitude, population improvement programme 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 sea makes use of non-additive gene effects; and reciprocal recurrent selection utilises both additive and non-additive gene effects.
(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.
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 heterozygotes into homozygotes. 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 out-breeders.
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.
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