it describes how non additive gene action occures

1. Gene Interaction: Non additive gene
action
Delivered on: 12/04/2018
Delivered by
Dipti Kujur
M.Sc.(Ag.) Previous Year
Dept. Of Genetics and Plant
Breeding
Presentation on

2. Contents
 Introduction
 Main features of gene action.
 Non additive gene action
 Dominance variance
 Epistatic variance
 Breeding procedure
 steps
 Factor affecting genetic variance.
 Case study

3. INTRODUCTION
 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
hybridization programmes and also in the choice of appropriate
breeding procedure for the genetic improvement of various
quantitative characters.
 Gene action was first studied by Archibald Edward Garrod (1902) in
human and subsequently by other in smaller organisms like Drosophila,
Neurospora and Bacteria who was an English Physician.

4. Main features of Gene action
 Gene action is measured in terms of components of genetic variance or combining
ability variances and effects.
 Gene action is of two types:
1. Additive gene action (fixable variation)
2. Non-additive gene action (Un fixable variation)
 Additive gene action includes additive genetic variance and additive x additive
type of epistatic variance.
 Non additive gene action includes :1. Dominance variance (d) or D
2. Epistatic variance
Additive x additive variance (i) or I
Additive x dominance (j) or J
Dominance x dominance (l) or L

5. Gene action can be studied with the help of various
biometrical techniques such as diallel analysis, partial
diallel cross, triallel analyis, quadriallel analysis, line x
tester analysis, generation mean analysis, biparental
cross and triple test cross analysis.

6. Dominance Action (D)
 It refers to the deviation from the additive scheme of gene action
resulting from intra-allelic interaction.
 It is due to the deviation of heterozygote (Aa) from the average of
two homozygotes (AA and aa).
 When d = (Aa-m) 0, gene A is showing dominance action.
 Depending upon the position of heterozygote in relation to m on
the hereditary scale : Complete,
Partial,
Overdominance.

7. 1. Complete dominance: When Aa = AA or aa, a complete
dominance of A over a (positive), or vice versa (negative)
reflected.
 Aa=AA, and Bb=BB, i.e, heterozygotes are equal to
homozygotes. Hence d 0 and therefore, additivity is
absent.
1. Partial Dominance: When Aa > m but <AA, or AA < m but
> aa, partial dominance of A over a (positive), or vice
versa (negative) is operative.
2. Over dominance: When Aa > AA or Aa < aa
overdominance of allele A over a (positive), or vice versa
(negative) is envisaged.

8.  It is a measure of dominance gene action.
 It is associated with heterozygosity &, therefore, it is
expected to be maximum in cross-pollinating crops and
minimum in self-pollinating species. It is not fixable &,
therefore, selection for traits is not fixable.
 It is chief cause of heterosis or hybrid vigour.
 Specific combining ability variance is the measure of
dominance variance in diallel, partial diallel and line x
tester cross analysis.
 Dominance variance gets depleted through selfing or
inbreeding.
 In natural breeding populations, dominance variance is
always lesser than additive variance.
Main features:

9. Epistatic (inter-allelic interaction) (I)
It refers to the deviation from additive scheme as a consequence of
inter-allelic interaction, i.e., interaction between alleles of two or
more different genes or loci.
Main features:
Epistatic variance includes both additive and non-additive
components.
It is of three types : Additive x Additive
Additive x Dominance
Dominance x Dominance
First type of epistasis is fixable and therefore, selection is effective
for traits governed by such variance.

10.  Last two type of epistatic variances are unfixable – heterosis
breeding may be rewarding.
 In case of generation mean analysis, the epistatic gene
interactions are classified on the basis of sign (negative or
positive) of (h) and (l) into 2 types: complementary
duplicate
• When (h) and (l) have the same sign, it is called complementary
type.
• When (h) and (l) have opposite sign, it is termed as duplicated
tpe of epistasis.
 In the natural plant breeding population, epistatic variance has
the lowest magnitude.

11. Breeding procedure to be followed
 Heterosis breeding
 Population improvement by recurrent selection for sca

12. Steps involve in gene action
1. Selection of genotypes.
2. Making crosses.
3. Evaluation of material.
4. Analysis of data.

13. 1. Selection of genotypes: include varieties, strains or germplasm lines.
2. Making crosses: The selected genotypes are crossed according to the
mating design to be used.
Choice of mating design depends on the type of genetic material. The
mating designs, diallel, partial diallel, and line x tester analysis are
commonly used for estimation of genetic variances from single crosses.
• Triallel analysis : used for estimation of genetic variances in three-
way crosses.
• quadriallel analysis : evaluation of double crosses.
• Triple test cross analysis provides information about the presence or
absence of epistasis in addition to the estimates of additive and
dominant components.
• Three biometrical techniques ,viz., generation mean analysis, triallel
analysis and quadriallel analysis provide information about all the
three components of genetic variance, viz., additive, dominance and
epistatic variances.

14. 3.) Evaluation of material:
The crosses made among selected genotypes are
evaluated along with parents in replicated trials and
observations are recorded on various quantitative
characters.
4) Analysis of Data:
The biometrical analysis of data is carried out as per
the mating design adopted. Diallel , partial diallel,
line x tester analysis and biparental cross analysis
provide estimates of additive and dominance
components of genetic variance.

15. Factor affecting gene action
1. Type of Genetic Material:
The magnitude of gene action is largely governed by the type of genetic material
used for study.
 In a F2 or advanced generation : the genetic variance includes additive,
dominance, and epistatic components.
 Homozygous lines : the genetic material is entirely of additive and additive –
epistatic types.
2) Mode of pollination:
The gene action is greatly influenced by the mode of pollination of a plant species.
 Self pollinated species : additive gene action is associated with homozygosity.
Inbreeding increase the amount of additive genetic variance in a population due to
increase , in homozygosity by way of gene fixation.
 Cross-pollinating crops : Dominance gene action is associated with heterozygosity.

16. 3)Mode of Inheritance:
Polygenic characters are governed by both additive and non-
additive type gene action , 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 type of gene action.
4) Sample size:
The 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.

17. Case study
Gene action of fruit yield and quality traits in okra (Abelmoschus esculentus (L.) Moench)
were studied through half diallel analysis of 28 F1 hybrids derived by crossing 8 parental
lines. The study indicated the preponderance of non-additive gene action for days to 50%
flowering, nodes per plant, fruit length, fruit diameter, plant height, fruits per plant and
mucilage and a preponderance of additive gene action for days to first picking, first fruit
producing node, internodal length, average fruit weight and harvest duration. For fruit
yield per plant and dry matter, only dominant component of variance was observed which
revealed the presence of non-additive gene action, hence, heterosis breeding is required to
be followed for exploitation of these traits.

18. The study was carried out to determine the type of gene action, genetic parameters of yield
and other quantitative traits by crossing 8 diverse maize inbred lines in complete diallel
fashion. Seed of F1 population along with their parents was planted in randomized complete
block design replicated thrice. The estimates of components of genetic variation revealed
that non additive genetic effects were more pronounced in the inheritance of plant height,
days to 50% tasseling, days to 50% silking, ear height and grain yield per plant. Directional
dominance was observed for all the characters under study. The graphic analysis showed
that all the characters were under the genetic control of over dominance type of gene action,
therefore, the material can easily be exploited for heterotic effect.