Gene interactions occur when two or more different genes influence the
outcome of a single trait
Epistasis is a phenomenon in which the expression of one gene depends on
the presence of one or more modifier genes.
A gene whose phenotype is expressed is called epistatic.
For example: If two epistatic genes A and B are mutated and each mutation
by itself produces a unique phenotype but the two mutations together show
the same phenotype as the gene A mutation then gene A is epistatic to gene
Epistasis can be contrasted with dominance, which is interaction between
alleles at the same gene locus.
Types of epistasis
Two genes may be required to produce the same effect.
e.g. flower color in sweet pea.
9 C_P_ : 3 C_pp :3 ccP_ : 1 ccpp
One gene may act as an inhibitor of the effect of another gene,
e.g. aleurone color in maize.
Parents: Red X White
Either of two genes may produce a similar effect or the same effect is
produced by both of them together.
e.g. seed capsule of bursa.
F1 (TtVv) x F1 (TtVv)
TV Tv tV tV
One gene has no visible effect unless a second gene is present at another
e.g. grain color in maize
Parents: Purple X White
F1: PpRr (purple)
Two genes may produce the same effect, but the effects are additive if both
genes are present.
e.g. awn in barley
Parents: Long awn X Awnless
F1: AaBb(long awn)
F2: 9A-B-:long awn
One gene may hide the effect of a second gene when both are present.
e.g. seed coat color in barley.
Parents: Black X Yellow