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.

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