Types and components of variation, phenotypic, genotypic and environmental variation, additive , dominance and epistatic variance

1. Sudha's KSM 95

2. Sudha's KSM 95

3. Nature of variability –
Types and components of
variation.
By:
Kalluru Sudha Mani
ADPM/17/2555Sudha's KSM 95

4. VARIABILITY:
refers to the presence of differences among the
individuals of plant population.
• Either due to differences either- in the genetic constitution,
- in the environment.
SOURCES OF VARIABILITY:
1.Spontaneous mutations,
2.Natural out crossing,
3.Recombinations .
Sudha's KSM 95

5. Types of polygenic variation:
• Phenotypic variation
• Genotypic variation
• Environmental variation
Sudha's KSM 95

6. PHENOTYPIC VARIATION:
• Variation due to changes in both the genotype and
environment which in turn affect the phenotype of the
individual.
• Also known as total variation.
• It changes under different environmental conditions.
• Measured in terms of phenotypic variance(P).
Sudha's KSM 95

7. ENVIRONMENTAL VARIATION:
• Non heritable variation which is entirely due to
environmental effects and varies under different
environmental conditions.
• This uncontrolled variation is measured in terms of Error
mean variance(E).
Sudha's KSM 95

8. GENOTYPIC VARIATION:
• Heritable or genetic variability which remains unaltered by
environmental conditions.
• More useful to a plant breeder for exploitation in selection
of hybridization.
• Measured in terms of genotypic/genetic variance(G).
• Consists of :
1.Additive variance
2.Dominance variance
3.Epistatic variance
Sudha's KSM 95

9. CLASSIFICATION OF GENETIC
VARIANCE
Sudha's KSM 95

10. ADDITIVE VARIANCE (A):
• Genetic variance due to average effects of genes on all
segregating loci.
• Arises from differences between two homozygotes of a gene i.e.,
AA and aa. Show lack of dominance and have intermediate
expression in Aa.
• Associated with homozygosity, maximum in self pollinated crops
and minimum in cross pollinated crops.
• Fixable and selection for traits governed by such variance is
effective.
Sudha's KSM 95

11. • Only variance responds to selection and depletes
proportionately to the improvement made by selection.
• Measures the :- Additive gene action
• Breeding value of a genotype
• Narrow sense heritability
• Measured by:- GCA of a parent
• TRANSGRESSIVE SEGREGATION.
• Highest in magnitude in natural population.
Sudha's KSM 95

12. DOMINANCE VARIANCE(D):
• Refers to the intra allelic interaction i.e., interaction b/w alleles
of same gene or locus i.e., deviation from the additive gene
action.
• No difference b/w homozygous dominant parent and
heterozygote, show incomplete , complete , over dominance.
• Associated with heterozygosity, max in cross pollinated crops
and min in self pollinated crops.
Sudha's KSM 95

13. • Not fixable and selection is ineffective.
• Gets depleted through selfing or inbreeding.
• Measured by: SCA in diallele, partial diallele and line x tester
cross analysis.
• HETEROSIS/HYBRID VIGOUR.
• Moderate in magnitude in natural population.
Sudha's KSM 95

14. EPISTATIC VARIANCE(I):
• refers to the inter allelic interaction i.e., interaction between
the alleles of two or more different genes or loci.
• Includes both additive and non additive components.
• It is of three types :- a) Additive x Additive
b) Additive x Dominance
c) Dominance x Dominance
• Lowest in magnitude in natural populations.
Sudha's KSM 95

15. Additive x Additive Additive x Dominance Dominance x Dominance
Interaction b/w loci each
exhibiting lack of
dominance individually.
Interaction b/w loci one
exhibiting lack of
dominance and the other
dominance individually.
Interaction b/w loci each
exhibiting dominance
individually.
Represented by A x A A x D D x D
Fixable Non fixable Non fixable
Selection is effective Heterosis is effective Heterosis is effective
Have additive gene
action
Non additive gene action Non additive gene action
Sudha's KSM 95

16. Sudha's KSM 95