Types of Gene Action Help
Alleles may interact with one another in a number of ways to produce variability in their phenotypic expression. The following models may help us understand various modes of gene action.
- With dominance lacking, i.e., additive genes, or a heterozygote that is intermediate between the two homozygotes, each A1 allele is assumed to contribute nothing to the phenotype (null allele), whereas each A2 allele contributes one unit to the phenotype (active allele).
- With partial or incomplete dominance, the heterozygote contributes almost as much as the A2A2 homozygote.
- In complete dominance, identical phenotypes are produced by the heterozygote and A2A2 homozygote.
- In overdominance, the phenotype of the heterozygote is beyond the range of either homozygote.
If allelic interaction is completely additive, a linear phenotypic effect is produced. In Fig. 8-5, a constant increment (i) is added to the phenotype for each A2 allele in the genotype.
Even if complete dominance is operative, an underlying component of additivity (linearity) is still present (solid line in Fig. 8-6). The deviations from the additive scheme (dotted lines) due to many such genes with partial or complete dominance can be statistically estimated from appropriately designed experiments. The genetic contributions from such effects appear in the dominance component of variance (σ2D).
In a much more complicated way, deviations from an underlying additive scheme could be shown to exist for the interactions between genes at different loci (epistatic relationships). The contribution to the total genetic variance (σ2G) made by these genetic elements can be partitioned into a component called the epistatic or interaction variance (σ2I).
The sum of the additive gene effects produced by additive genes and by the additive contribution of genes with dominance or epistatic effects appears in the additive component of genetic variance (σ2A).
Thus, the total genetic variance can be partitioned into three fractions:
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