Application of Biometrical techniques in Crop Improvement ?

Biometrical techniques are useful to plant breeder in four principal ways, viz., 

(1) Assessment of polygenic variability, 

(2) selection of elite genotypes, 

(3) Choice of parents and breeding procedure

(4) Study of varietal adaptation. These are briefly, discussed below.

1. Assessment of Polygenic Variation

The development of an effect plant breeding programme is dependent upon the existence of genetic variability. The efficiency of selection largely depends upon the magnitude of genetic variability present in the plant population. Thus the success of genetic improvement in any character depends on the nature of variability present in the gene pool for that character. Hence an insight into the magnitude of variability present in the gene pool of a crop species is of utmost importance to a plant breeder for starting a judicious plant breeding programme. In earlier years the visual observation used to be the measure of variability in a plant population. Now biometrical methods are available for systematic assessment of variability (Frey, 1966). There are four biometrical techniques, viz. simple measures of variability (range mean, standard deviation, variance, standard error, coefficient of variation components of genetic variance, metroglyph analysis and D2 statistics which are commonly used for the assessment of polygenic variability in plant breeding populations.

The simple measures of variability especially the coefficient of variation partitions the total variation into phenotypic, genotypic and environmental components and determines the magnitude of these components for various traits. The study of components of genetic variance helps in further partiationing of genetic variance into additive, dominance and epistatic components. A magnitude of these components is a measure of the type of gene action involved in the expression of various traits. Information about gene action helps in deciding a breeding procedure for the genetic improvement of a trait. The D² statistics evaluates large number of germplasm lines for genetic diversity and helps in the identification of genetically divergent parents for their exploitation in hybridization programmes. Meteroglyth analysis is a simple technique for evaluation of phenotypic variability in large number of germplasm lines at a time.

2. Selection of Elite Genotypes

Selection is perhaps the most important activity in all plant breeding programmes. Various types of selection schemes viz., mass selection, progeny selection and cyclic selection, are used depending on the mode of pollination of crop species, the predominant gene action and breeding objective, Selection is practised both in homozygous as well segregating populations. The efficiency of selection largely depends on extent of genetic variability present in the population, and the heritability of concerned character Selection is generally more effective for characters of high heritability than those having low heritability. High yield is the prime objective in all breeding programmes, but generally yield has low heritability and direct selection is not sufficiently effective. Hence it is desirable to select indirectly for improved yield. Some biometrical techniques provide information about the relative contribution of various component traits towards yield and aid in the selection of superior genotypes from the breeding populations. Such techniques include correlations, path analysis and discriminant function analysis. Correlation measures the mutual relationship among various plant characters and helps in determining the yield components on which indirect selection can be based for improvement in yield. Path analysis splits the correlation coefficient into the measures of direct and indirect effects and determines the direct and indirect contribution of various characters towards yield. Discriminant function helps in the identification of character combination having high selection efficieny than direct selection for yield.

3. Choice of Parents and Breeding Procedures

Hybridization is the most potent technique for breaking yield barriers and evolving varieties having built-in high yield potential. The selection of suitable parents for hybridization is one of the most important steps in a breeding programme. Selection of parents on the basis of phenotypic performance alone is not a sound procedure since phenotypically superior lines may yield poor recombinants in the segregating generation. It is therefore, essential that parents should be chosen on the basis of their genetic value. There are several biometrical techniques for the evaluation of varieties or strains in terms of their genetic makeup. Three biometrical fechniques, viz., generation mean analysis, biparental cross analysis and triple test cross analysis are used for the analysis of individual cross in terms of components of genetic variance. Another three biometrical techniques namely diallel cross, partial diallel cross and line X tester analysis are used for evaluation of several single crosses simultaneously: Triallel analysis is used for evaluating several threeway crosses and quadriallel analysis is adopted for the simultaneous evaluation of several double crosses. In crop improvement three biometrical techniques, namely diallel, partial diallel and line x tester analyses are commonly used in the selection of parents for hybridization. The parents are chosen on the basis of their combining ability and the breeding procedure is decided on the basis of gene action involved in the expression of various quantitative characters.

4. Assessment of Varietal Adaptability

Varietal adaptability to environmental fluctuations is important for the stabilization of crop production both over regions and years. Estimation of phenotypic stability, which involves regression analysis has proved to be a valuable technique in the assessment of varietal adaptability. Stability analysis is useful in the identification of adaptable genotypes and in predicting the response of various genotypes over changing environments. There are different stability analysis models for this purpose, such as Finlay and Wilkinson (1963) model, Eberhart and Russell (1966) model, Perkins and Jinks (1968) model, and Freeman and Perkins (1971) model. It is generally agreed that the more stable genotypes can some how adjust their phenotypic responses to provide some measure of uniformity in spite of environmental fluctuations. The buffering ability of the segregating populations seems to be directly related with the homeostatic responses (buffering capacity) of the parental lines. Therefore, it is feasible to develop phenotypically stable high potential genotypes by incorporation of homeostatic genotypes in the hybridization programme.

Advantages of Biometrical Techniques

Biometrical techniques provide basic information about various genetical aspects, which is useful for better understanding of the subject as well as better planning of plant breeding programmes. Thus Quantitative genetics is useful in several ways as given below:

1. Quantitative genetics provides solution to the analysis of polygenic characters, which is not possible through Mendelian genetics.

2. Quantitative genetics utilizes various statistical models and designs for the analysis of polygenc characters.

3. Quantitative genetics provides ways and means for genetic evaluation of various plant populations in terms of variability, heritability, combining ability, gene action, yield attributes, adaptation, etc.

4. Quantitative genetics helps in the separation of non-heritable variation from heritable variation, which is not possible in Mendelian genetics.

5. Quantitative genetics provides statistical bases for scientific interpretation of results in plant breeding research.

Limitations of Biometrical Techniques

Quantitative genetic studies have several useful applications in plant breeding and genetics, but have some limitations also which are briefly described below

1. Quantitative genetic studies are not applicable to qualitative data like colour, shape, form, etc., unless these are converted into quantitative figures.

2. Quantitative genetic studies are always based on mean values. Individual data are not given importance in most of the analysis.

3. Since the plant characters are sensitive to environmental variations, quantitative genetic estimates are also valid for a particular set of environmental conditions and these estimates may change with the change in the environment.

4. The interpretation of Quantitative genetic estimates requires high technical skill. Improper interpretation of estimates may sometimes give misleading information.

5. Quantitative genetic estimates are influenced by several factors like, type of genetical material, size of sample, sampling method, conduction of experiment, method of calculation, effect of linkage, etc.