Everything you need to know about genetic variance !
Q. 1. What is genetic variance?
Ans. The heritable portion of phenotypic (total) variance is called genetic variance. It is of three types, viz., additive, dominance and epistatic variances.
Q. 2. What is additive variance?
Ans. Average effects of genes on all segregating loci refer to additive variance. It is the component which arises from differences between two homozygotes of a gene i.e. AA and aa.
Q. 3. What are main features of additive variance?
Ans. Main features of additive genetic variance are given below:
(i) Additive genes show lack of dominance i.e. intermediate expression.
(ii) Additive variance is fixable.
(iii) Transgressive segregation is the result of additive gene action.
(iv) The gca is a measure of additive variance.
(v) Additive variance responds to selection.
Q. 4. What is dominance variance?
Ans. The deviation from the additive scheme resulting from intra-allelic interaction is called dominance variance. It is due to deviation of heterozygote (Aa) from the mean of two homozygotes (AA and aa).
Q. 5. What are main features of dominance variance?
Ans. The main features of dominance variance are given below:
(i) The heterozygote resembles to one of the two parents.
(ii) Dominance variance is not fixable.
(iii) It is associated with heterozygosity.
(iv) The sea is a measure of dominance variance.
(v) Selection is not effective with dominance variance.
(vi) It is the chief cause of heterosis.
Q. 6. What is epistatic variance?
Ans. Deviation from the additive scheme as a consequence of inter-allelic interaction.
The main features of epistatic variance are given below:
(i) It includes both additive and non-additive components.
(ii) It is of three types, viz. additive x additive, additive x dominance, and dominance x dominance.
(iii) Additive x additive type of epistasis is fixable and rests are non-fixable.
Q. 7. What is additive x additive epistasis?
Ans. It refers to interaction between two or more loci, each exhibiting lack of dominance individually. It is denoted as A x A and is fixable. It comes under additive gene action. Selection is effective for traits governed by this type of epistatic variance’.
Q. 8. What is additive x dominance epistasis?
Ans. It refers to interaction between two or more loci, one exhibiting lack of dominance and the other dominance individually. It is denoted as A x D and is non-fixable.
Q. 9. What is dominance x dominance epistasis?
Ans. It refers to interaction of two or more loci, each exhibiting dominance individually. It is represented as D x D and is non-fixable.
Q. 10. What is gene action?
Ans. The behaviour or mode of expression of genes in a genetic population is called gene action. Gene action is of two types, viz. additive gene action and non-additive gene action.
Q. 11. What is additive gene action?
Ans. Joint effects of additive variance and additive x additive type of epistasis refer to additive gene action.
Q. 12. What is non-additive gene action?
Ans. It includes dominance variance and additive x dominance, and dominance x dominance types of epistasis.
Q. 13. What is fixable variance?
Ans. It refers to gene action/genetic variance which can be fixed as a true breeding line. It includes additive variance and additive x additive type of epistasis.
Q. 14. What is non-fixable variance?
Ans. The genetic variance which cannot be fixed as a true breeding line is called non-fixable variance. It included dominance variance and additive x dominance and dominance x dominance types of epistatic variances.
Q. 15. What is classification of genetic variance according to Fisher (1918)?
Ans. Fisher (1918) divided genetic variance into three components, viz. additive, dominance and epistatic variances.
Q. 16. What are components of genetic variance as per Wright (1935)?
Ans. Wright (1935) classified genetic variance into two components, viz. additive variance and non-additive variance. Non-additive’ variance includes dominance variance and A x D and D x D types of epistasis.
Q. 17. What are components of genetic variance as per Mather (1949)?
Ans. Mather (1949) divided genetic variance into two components, viz. heritable fixable and heritable non-fixable. The heritable fixable variance is, additive variance and heritable non-fixable variance refers to non-additive variance.
Q. 18. What is the genetic constitution of natural breeding populations?
Ans. In natural plant breeding populations, additive variance is predominant followed by dominance variance and epistatic variance,
Q. 19. What is genetic constitution of F2 population?
Ans. F2 population has ½ additive genetic variance and ¼ dominance variance.
Q. 20. What type of gene action is found in random mating populations?
Ans. Random mating populations such as composite and synthetic varieties have both additive and non-additive gene actions.
Q. 21. Which breeding methods would be rewarding when a character is governed by additive gene action?
Ans. In self pollinated species, if a character in governed by additive gene action three breeding procedures, viz. pure line selection, mass selection and progeny selection would be rewarding.
Q. 22. Which breeding methods would be rewarding in cross pollinated species, if a character is governed by additive gene actor?
Ans. Under such situation recurrent selection for gca, synthetic and composite breeding methods would be rewarding.
Q. 23. Why selection is not effective in a pure line?
Ans. In a pure line, further improvement through selection is not possible because additive genetic variance is completely depleted. The pure line population has exactly the same genetic constitution. The variation is due to environmental factors.
Q. 24. How variation is generated in a pure line?
Ans. In a pure line over a period of time, additive variance is regenerated by natural out-crossing and spontaneous mutation.
Q. 25. When additive and non-additive gene actions are of equal magnitude, which breeding method would be effective?
Ans. When a character is governed by both, additive and non- additive gene actions, reciprocal recurrent selection would be rewarding for population improvement.
Q. 26. Which breeding method would be rewarding when a character is governed by non-additive gene action?
Ans. When a character is governed by non-additive gene action, heterosis breeding would be rewarding both in self- and cross-pollinated species.
Q. 27. In F1 population, what type of gene action is expected?
Ans. In F1 population, usually non-additive (dominance and epistasis) gene action is expected.
Q. 28. What type of gene action is expected in cross pollinated species?
Ans. In a cross pollinated species, both additive and non-additive (additive, dominance and epistatic variances) gene actions are expected.
Q. 29. What type of gene action is expected in self pollinated species?
Ans. In self pollinated species, additive gene action (additive variance and additive x additive type of epistasis) are expected.
Q. 30. What biometrical techniques are used for the study of gene action?
Ans. The gene action can be studied with the help of following biometrical techniques:
(i) Diallel cross analysis
(ii) Partial diallel analysis
(iii) Line x tester analysis
(iv) Generation mean analysis
(v) Triallel cross analysis
(vi) Quadriallel cross analysis
(vii) Triple test cross analysis
(viii) Biparental cross analysis.
Q. 31. What are the factors affecting gene action?
Ans. The gene action is influenced by the following factors:
(i) Type of genetic material used for study
(ii) Mode of pollination of a species
(iii) Mode of inheritance of the character
(iv) Presence or absence of linkage
(v) Sample size and sampling method
(vi) Method of calculation.
Q. 32. What is the relationship between gene action and heritability?
Ans. Narrow sense heritability has positive association with additive gene action. Higher the value of additive genetic variance, higher the estimates of heritability in narrow sense and vice versa.
Q. 33. What is the relationship of gene action with heterosis?
Ans. There is positive association between non-additive variance and magnitude of heterosis. The presence of significant amount of non-additive variance is a pre-requisite for commercial exploitation of heterosis.
Q. 34. How gene action is affected by mode of pollination?
Ans. The additive gene action is associated with homozygosity, and therefore, it is expected to be the maximum in self pollinated species. The non-additive gene action is associated with heterozygosity and, therefore, it is expected to be maximum in cross pollinated species and minimum in self pollinated species.
Q. 35. What is the role of gene action in plant breeding?
Ans. The knowledge of gene action to a plant breeder is useful in following three main ways:
(i) In the selection of suitable parents for hybridization.
(ii) In the choice of breeding procedures for the genetic improvement of various quantitative characters.
(iii) In the estimation of some other genetic parameters such as heritability, degree of dominance etc.