Table 9 Relationship between two-locus epistasis and the maternal-effect model
Locus B
B1B1B1B2B2B2
A. Two-locus epistasis with unordered genotypesa
Locus A
 A1A1+aA +aB + aa+aA + dB + ad+aAaBaa
 A1A2+dA +aB + da+dA +dB + dd+dAaBda
 A2A2aA + aBaaaA + dBadaAaB + aa
Offspring genotype
A1A1A1A2A2A2
B. Maternal-effect modelb
Maternal genotype
 A1A1+am +ao + aamo+am + do + admo
 A1A2+dm +ao + damo+dm +do + ddmo+dmaodamo
 A2A2am + doadmoamao + aamo
  • a With two-locus epistasis with unordered genotypes, epistasis can be measured as the interaction between the additive (aA and aB) and dominance effects of the two loci (dA and dB). There are four forms of epistasis corresponding to the additive-by-additive (aa), additive-by-dominance (ad), dominance-by-additive (da), and dominance-by-dominance (dd) interaction effects (Cheverud 2000).

  • b With the maternal-effect model, the cells are maternal–offspring genotype combinations at a single locus (Wade 1998; Wolf and Hager 2006), so the interactions are between the additive (am) and dominance (dm) maternal effects with the additive (ao) and dominance (do) direct effects at the same locus. There are two missing genotype combinations (dashes) because mothers that are homozygous for one allele cannot have offspring that are homozygous for the other allele.