  Dominance is usually considered a constant value that describes the relative
difference in fitness or phenotype between heterozygotes and the average of
homozygotes at a focal polymorphic locus. However, the observed dominance can
vary with the genetic background of the focal locus. Here, alleles at other
loci modify the observed phenotype through position effects or dominance
modifiers that are sometimes associated with pathogen resistance, lineage, sex,
or mating type. Theoretical models have illustrated how variable dominance
appears in the context of multi-locus interaction (epistasis). Here, we review
empirical evidence for variable dominance and how the observed patterns may be
captured by proposed epistatic models. We highlight how integrating epistasis
and dominance is crucial for comprehensively understanding adaptation and
speciation.
