This document discusses genetic components of polygenic variation and breeding strategies. It describes three types of genetic variation: phenotypic, genotypic, and environmental. Polygenic variation is controlled by multiple genes and includes these three components. Genotypic variation is the inherent genetic variation and is most useful for plant breeders. It consists of additive, dominance, and epistatic components. Additive variation is caused by average gene effects and is fixable through selection. Dominance variation is due to interactions between alleles and is not fixable. Epistatic variation results from interactions between genes. The document outlines the different models used to classify genetic variation and explains the implications of each component for plant breeding.

1. Course No.: AGP-122
Dr. D. K. Singh
Deptt. of Genetics & Plant Breeding
College of Horticulture
BUAT, Banda (U. P.)-210001
Genetic Components of Polygenic Variation
and
Breeding Strategies

2. Components of Genetic Variation
Variation:- Any observable variability found between two living organism.
Types of Variation
1.Phenotypic Variation. The total variability which is observable.
2.Genotypic Variation. The inherent or genetic variation which remains
unaltered by environmental changes.
3.Environmental Variation. The non-heritable variation which is entirely due
to environmental effects and varies under different environmental
conditions.
Causes of Variation by genes:-
Monogenic Variation:- One or two Distinct dominant genes leads to bring
change in trait expression. Eg. petal colour of flowers.
Polygenic variation:- When one characteristic is controlled by two or more
genes. Often the genes are large in quantity but small in effect. Examples
of human polygenic inheritance are height, skin color, eye color and weight.
The polygenic variation present in a plant population is of three types, viz.,
phenotypic, genotypic and environmental.

3. Phenotypic variability is observable. It includes both genotypic and
environmental variation and, therefore, is also called total variation.
It changes under different environmental conditions and is
measured in-terms of phenotypic variance.
Genotypic variation refers to genetic or inherent
variability which remains unaltered by environmental conditions.
This type of variability is more useful to a plant breeder for
exploitation in selection or hybridization. It is measured in terms of
genotypic variance and consists of additive dominance and epistatic
components (see later).
Environmental variation is entirely due to environmental
effects and varies under different environmental conditions. This
uncontrolled variation is measured in terms of error mean variance.
The variation in purelines and their F is non- heritable
(environmental).

4. Fisher (1918),
Additive,
Dominance
Epistatic
Av. Effects on genes.
Dev. Fr Mean value, intra-allelic
Dev. Fr Mean value inter-allelic
3 types(AA,AD,DD)
Wright (1935)
Additive
variance
Non-additive
variance.
Same as given by fisher
Includes Dominant & Epistatic variances
Mather (1949).
Heritable
fixable
Heritable non-
fixable
Includes additive var. & AA component of
Epistasis.
Includes dominant var. & AD & DD
component of Epistasis
Genetic Variance. Heritable portion of total or phenotypic variance. It is of
three types, viz. additive, dominance and epistatic Variances
Table 1. Classification of genetic variance according to various Authors.

5. • In crop improvements programme, only the genetic components of variation are important
because only this component is transmitted to the next generation.
• According to Fisher in 1918, components of genetic variance divided into three
components viz.
• 1) Additive, 2) Dominance 3) Epistatic
1) Additive Components:
• It is the component arising from difference between the phenotypes of the two
homozygotes for a gene, Eg. AA and aa.
2) Dominance Component:
• It is due to the deviation of heterozygote (Aa) phenotype from the average of
phenotypic value of the two homozygotes (AA and aa). It is also referred as intra-allelic
interaction.
3) Epistatic or Interaction Components:
• It results from an interaction between two or more genes.
• Later Hayman and Mather classified the epistatic components into three types
interaction viz.
• Additive X Additive, Additive X Dominance, Dominance X dominance.
Role of Genetic Variance:

6. Additive variance (A):- It refers to that portion of genetic
variance which results due to average effects of genes on all
segregating loci. Thus it is the component which arises from
differences between two homozygotes of a gene, i.e., AA and aa.
Main features of additive genetic variance are given below:
It is a measure of additive gene action. show lack of dominance,
i.e., intermediate expression.
It is associated with homozygosity ,max. in self-pollinating crops
and minimum in cross.
It is fixable and therefore, selection for traits governed by such
variance is very effective.
It is required for estimation of heritability in narrow sense and
response to selection is directly proportional to narrow sense
heritability.

7. Breeding value of an individual is measured directly by the
additive gene effects. The general combining ability (GCA) effect
of a parent is a measure of additive gene effects.
It gets depleted proportionate to the improvement made by
selection.
In natural plant breeding populations, additive variance is the
predominant one closely followed by dominance variance.
Additive gene action is the chief cause of resemblances between
relatives and progeny.
Transgressive segregants is the result of additive
gene action, which fall outside the range of parents, are obtained
due to fixation of dominant and recessive genes in separate
individuals, Such segregation occurs when the parents are
intermediate to the extreme values of the, Segregating population.

8. Dominance Variance (D):- It refers to the deviation from the
additive scheme of gene action resulting from intra-allelic
interaction, i.e., interaction between alleles of the same gene or
same locus.
It is due to the deviation of heterozgote (Aa) from the average
of two homozygotes (AA and aa).
Main features of dominance variance are briefly discussed below:
It is a measure of dominance gene action. Such genes show
incomplete dominance, compelete dominance or overdominance.
In other words, the heterozygote (Aa) does not represent
mean value of two parents for a particular character. The parent
to which the heterozygote approaches more closely for a
particular trait is known as dominant parent and the other as
recessive parent.

9. Dominance variance is associated with heterozygosity and,
therefore, it is expected to be maximum in cross-pollinating
crops and minimum in self-pollinating species.
Dominance variance is not fixable and, therefore, selection for
traits controlled by such variance is not effective.
Dominance variance is the chief cause of heterosis or hybrid
vigour.
Specific combining ability variance(SCA) 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.

10. Epistatic variance (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 of epistatic variance are given below:
Epistatic variance is a measure of epistatic gene action.
Epistatic variance is of three types, viz. additive x additive,
additive x dominance and dominance x dominance as defined below
:
Additive x additive.(A X A) 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.
Additive x dominance.(A X D) 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.

11. Dominance x dominance.(D X D):- It refers to interaction of two
or more loci, each exhibiting dominance individually. It is
represented as D x D and is non-fixable.
First type of epistasis is fixable and, therefore, selection is
effective for traits governed by such variance,
The last two of epistatic variances are unfixable and, therefore,
heterosis breeding may be rewarding for traits exhibiting such
variance.
In case of generation mean analysis, the epistatic gene
interactions are classified on the basis of sign of (h) and(l) into two
types. When (h) and (l) have the same sign, it is called
complementary type and when (h) and (I) have opposite sign, it is
termed as duplicate type of epistasis.
In natural plant breeding populations, epistatic variance has the
lowest magnitude.

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