EPISTASIS
When two or more genes influence a traits, an allele of one of them may have an overriding effect on the phenotype.When an allele has such an overriding effect is said to be epistatic to the other genes that are involved

TYPES
There are two types of epistasis

1:- Recessive Epistasis
2:-Dominant Epistasis
Recessive Epistasis:

A classic example of recessive epistasis from the work of Bateson and Punnett ,who studied the genatic control of flower colour in the sweet pae(lathyrus odoratus) The flowers in this plant are either purple or white ,purple if they contain anthocyanin pigment and white if they do not.
Bateson and Punnett crossed two different varients with white flowers to obtained F1 hybrids which all had purple flowers.When these hybrids were intercrossed,Bateson and Punnett obtained a ratio of 9purple:7white plants in F2.
Batson and Punnett work established that each of the recessive alleles are epistic over the dominent allele of the other gene .It showes that the production of anthocynin is controled by each domint allele when that is absent no pigments are produced and the flower remain white(recessive).

P1
White white
CCpp x ccPP
F1
Purple Purple
CcPp x CcPp

F2

summary: 9/16 purple, 7/16 white

Dominant epistasis
Another study of domenent epistasis was performed by George Shull using a weedy plant called Shepherd’s purse, Bursa bursa-pastoris,its all parts are edible and member of (brassicacea faimly) the seed capsules of this plant are either triangular or ovoid shape.ovoid capsules are produced only if a plant is homozygous for the recessive alleles of two genes that is if it has the genotype aa bb.if the dominant allele of either gene is present , the plant produces triangular capsules.the evidence for this conclusions comes from cross between doubly heterozygous plants.such crosses produce progeny in ratio of 15triangular:1ovoid indicating that the dominant allel of one gene is epistasic over the recessive allele of the other.

P
Tringular ovoid
AABB X aabb
F1
Tringular ovoid
AaBb X AaBb

F2

summary :15/16triangular, 1/16 ovoid

The data suggest that capsule shape is determined by duplicate developmental pathways, either of which can produce a triangular capsule.One pathway involves the dominent allele of the A gene, and the other the dominent allele of the B gene.A precursor substance can be converted into a product the leads to a triangular seed capsule through either of these pathways.Only when both pathways are blocked by homozygous recessive allele is the triangular phenotype and an ovoide capsule produced.