Lecture: - 7
Components of Genetic Variation: Heritability
and Genetic Advance
7.1 Genetic variance:-It refers to the heritable portion of total variance or phenotypic
variance.
➢ It is most useful variability and exploitable by the breeder.
➢ It is measured in terms of genotypic variance .
➢ It consist of additive, dominance and epistatic components (G=D+H+I)
➢ According to Fisher(1918):- It is of 3 types, such as;Additive variance, Dominance
variance, Epistatic variance.
7.2 Additive variance:-It refers to that portion of genetic variance which result due to
average effects of genes on all segregating loci.
➢ Arises of from difference between two homozygotes for a gene i.e. AA and aa.
➢ It is generally represented by ‘d’.
➢ It is due to lack of dominance.
Features:
• It is a measure of additive gene action. The additive gene show lack of dominance,i.e.
intermediate expression.
• 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.
• Additive variance is fixable and therefore selection for traits governed by such variance is
very effective.
• Additive variance is required for estimation of heritability in narrow sense and response t
selection is directly proportionate to narrow sense heritability.
• 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.
• Transgressive segregation is the result of additive gene action.
Components of Genetic Variation: Heritability
and Genetic Advance
7.1 Genetic variance:-It refers to the heritable portion of total variance or phenotypic
variance.
➢ It is most useful variability and exploitable by the breeder.
➢ It is measured in terms of genotypic variance .
➢ It consist of additive, dominance and epistatic components (G=D+H+I)
➢ According to Fisher(1918):- It is of 3 types, such as;Additive variance, Dominance
variance, Epistatic variance.
7.2 Additive variance:-It refers to that portion of genetic variance which result due to
average effects of genes on all segregating loci.
➢ Arises of from difference between two homozygotes for a gene i.e. AA and aa.
➢ It is generally represented by ‘d’.
➢ It is due to lack of dominance.
Features:
• It is a measure of additive gene action. The additive gene show lack of dominance,i.e.
intermediate expression.
• 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.
• Additive variance is fixable and therefore selection for traits governed by such variance is
very effective.
• Additive variance is required for estimation of heritability in narrow sense and response t
selection is directly proportionate to narrow sense heritability.
• 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.
• Transgressive segregation is the result of additive gene action.
