Genotypic variance
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- The variances are obtained by squaring the values multiplying by the frequency of the genotypes concerned and summing over the all genotypes.
- Genotypic variance is due to additive effects of genes and non-additive effects of genes.
- Additive effects are connected with breeding values of the individual because parents pass their genes to their offspring not their genotype.
Additive and Dominace variance
- Variance is obtained by squaring the values then multiplying by the frequency of the genotype concerned, and summing the three genotypes.
Genotype
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A1 A1
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A1 A2
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A2 A2
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Frequencies
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p2
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2pq
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q2
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Assigned values
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a
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d
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-a
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Breeding values
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2q a
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(q-p) a
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-2p a
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Dominance deviation
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-2q2 d
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2pqd
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-2p2 d
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- The additive variance which is the variance of breeding value is obtained as follows
- VA = (2q a )2p2 + [(q-p) a ]22pq + (-2p a )q2
- VA = 4p2q2 a 2 + (q2-2pq+p2) a 2 2pq + 4p2q2 a 2
- VA = 2pq a 2 (2pq + q2 - 2pq +p2 + 2pq)
- VA = 2pq a 2 (1)
- VA = 2pq [a + d (q-p)] 2
- The variance of dominance deviation is
- VD = (-2q2d) 2 + (2pqd) 2 + (-2p2d) 2
- VD = (2pqd) 2
- Total genetic variance
- VG = VA + VD
- VG = 2pq [a + d (q-p)] 2 + (2pqd) 2
- VG = 2pq a 2 + (2pqd) 2
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In general the genes contribute much more variance when at intermediate frequencies than when at high or low frequencies, recessives at low frequency, in particular, contribute very little variance.
Interaction variance
- When more than one locus is under consideration then the interaction deviations give rise to the interaction variance.
- It is the variation of the interaction deviations brought about by the epistatic interactions at different loci.
- Two factor interactions arise from interaction of two loci three factor from three loci etc.
- In two factor interaction
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Last modified: Saturday, 24 December 2011, 8:57 AM