ASSESSMENT OF VARIABILITY AND GENETIC VARIANCE
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This document discusses genetic variance and gene action. It defines additive, dominance, and epistatic variance as components of genetic variance. Additive variance is due to average gene effects and is fixable through selection. Dominance variance is due to interactions between alleles and is not fixable. Epistatic variance includes additive and non-additive interactions between loci. The document also discusses assessing variability through components of genetic variance estimated using diallel crosses and generation mean analysis.
ASSESSMENT OF VARIABILITY AND GENETIC VARIANCE
- 1. INDIRA GANDHI KRISHI VISHWAVIDYALAYA, RAIPUR AN ASSINMENT ON : ASSESSMENT OF VARIABILITY AND GENETIC VARIANCE Session –2015-16 SUBMMITED TO SUBMMITED BY Dr.Roshan parihar SHEKH ALISHA Dept.of Plant Breeding BSC(ag.)2nd year/2nd sem.
- 2. HIGHLIGHT Gene action Genetic variance Additive variance Dominance variance Epistatic variance Assessment of variability Simple measure Component of variance
- 3. GENE ACTION Gene action is refer to behavior 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 programmers and also in the choice of appropriate breeding procedure for the genetic improvement of various quantitative characters. FEATURES OF GENE ACTION 1.Gene action is measured in terms of components of genetic variance or combining ability variances and effect. 2. Gene action is two types i. e. additive gene action and non additive gene action 3. Gene action can be studied with the help of various biometrical techniques such as diallel analysis, partial diallel cross , line tester analysis , generation mean analysis
- 4. FACTOR AFFECTING GENE ACTION Types of genetic material Mode of pollination Mode of inheritance Existence of linkage Sample size Sampling method
- 5. GENETIC VARIANCE Genetic variance refers to the heritable portion of total variance or phenotypic variance. Genetic variance has been classified by Classified by types of genetic variance Fisher (1918) Wright (1935) Mather (1949) Additive variance Dominance variance Epistatic variance Additive variance Non additive variance Heritablefixable Heritable non fixable
- 6. ADDITIVE VARIANCE It refers to that portion of genetic variance which result due to average effects of genes on all segregating loci . Thus it the component which arises from differences between two homozygotes of a gene, i.e. AA and aa. FEATURES 1. It is a measure of additive gene action. The additive gene show lack of dominance,i.e. intermediate expression. 2. Additive genetic variance is associated with homozygosity and, therefore, it is expected to be maximum in self pollinating crops and minimum in cross- pollinating crops. 3. Additive variance is fixable and therfore selection for traits governed by such variance is very effective . 4. Additive variance is required for estimation of heritability in narrow sense and response to selection is directly proportionate to narrow sense heritability.
- 7. 5. Breeding value of an individual is measured directly by the additive gene effect . The GCA effect of a parent is a measure of additive gene effects. 6. Transgressive segregation is the result of additive gene action. DOMINANCE VARIANCE It refers to the deviation from the additive gene action resulting from intra-allelic ineraction,i.e. intraction between alleles of the same gene or same locus.it is due to deviation of heterozygotes (Aa) from the average of two homozygotes (AA and aa).
- 8. FEATURES 1. It is a measure of dominance gene action. Such gene show incomplete dominance, complete dominance or over dominance. In other words the heterozygote (Aa) does not represent mean value of two parent (AA and aa). It approaches more closely to one of the two parents for a particular character. 2. Dominance variance is associated with heterozygosity and therefore it is expected to be maximum in cross pollinating crops and minimum in self-pollinating species. 3. Dominance variance is not fixable and, therfore selection for traits controlled by such variance is not effective . 4. Dominance variance is the chief cause of heterosis or hybrid vigour. 5. Specific combination ability variance is the measure of dominance variance in diallel , partial diallele and line in
- 9. To tester cross analysis. 6. Dominance variance gets depleted throught selfing or inbreeding. 7. In natural breeding populations, dominance variance is always lesser than additive variance. EPISTATIC VARIANCE It refers to the deviation from additive scheme as a consequence of inter-allelic interaction, i.e.,intraction between alleles of two or more different genes or loci. FEATURES 1. Epistatic variance includes both additive and non additive components. 2. Epistatic variance is of three types viz, AA, AD and DD
- 10. ADDITIVE *ADDITIVE It refers to interaction between two or more loci each exhibiting lack of dominance individually. It is denoted as A*A and is fixable. It comes under additive gene action. ADDITIVE *DOMINANCE It refers to interaction between two or more loci, one exhibiting lack of dominance and the other dominance individually. it is denoted as A*D and is non fixable. DOMINANCE*DOMINANCE It refers to interaction of two or more loci, each exhibiting dominance individually. It is represented as D*D and is non fixable. 3. First type of epistatic is fixable and, therefore, selection is effective for traits governed by such variance. The last two types of epistatic variances are unfixable and,therefore ,heterosis breeding may be rewarding for
- 11. Traits exhibiting such variance. 4.In the natural plant breeding population, epistatic variance has the lowest magnitude. ASSESMENT OF VARIABILITY The variability present in breeding population can be assessed in four different ways; Using simple measure of variability Components of genetic variance By metroglyph analysis By D2 statistics
- 12. COMPONENTS OF GENETIC VARIANCE The magnitude of variation may also be assessed by estimating components of genetic variance. this involves crossing of a number of genotypes or strains in a definite fashion and evaluation progenies thus obtained in replicated trials. Diallel, partial diallel, line *tester generation mean analyses are commonly use.
- 13. Reference Genetic Basis and Methods of SULRAN SINGH PLANT BREEDING I.S.PAWAR PLANT BREEDINB Principles and Methods B.D.SINGH