What is an epistasis event?
An epistasis event is when "two separate genes (at different genetic loci) interact to affect the expression of a single trait." Source
Hypostatic: the mutation that is reliant on another to express. Its expression is being altered by another mutation.
Epistatic: the mutation that can alter the expression of another mutation. It is the determining factor in whether the other mutation can express.
"There are a couple different ways this can manifest:
one gene masking the effect of the other
both genes being required to produce a certain trait
one gene modifying the expression of another
(Other types of interaction are possible.)"
Note: The dominant or recessive nature of the mutations involved does not affect the epistasis event itself. Epi- and hypostatic mutations can be any combination of recessive or dominant.
Example 1: Albino
Albino alters the expression of the coat color despite other genes also coding for coat color. It is epistatic to all other coat color mutations, and it does not allow them to express. All other coat colors are hypostatic to albino, but not coat types.
Example 2: C dilutes & Splashed
Alleles of the C locus (AKA C dilutes) are part of an epistasis event with Splashed.
C dilutes, except for homozygous albino, are epistatic to splashed. They must express for splashed to express as well. This means that you can never have splashed express on anything except for C dilutes. This makes splashed hypostatic to C dilutes.
A list of C dilutes splashed can occur on:
c/c-e = Ivory
c/c-h = Himalayan
c/c-ch = Coffee / Light Mock Choc / Stone
c-e/c-e = Beige
c-e/c-h = Color Point Beige
c-e/c-ch = Mock Choc
c-h/c-h = Siamese
c-h/c-ch = Burmese
c-ch/c-ch = Silver Agouti/Sepia/Black Fox
Example 3: Abyssinian
A recent 2023 paper has described abyssinian, also called rosette, in a novel way. It was previously thought to be a simple recessive, but this is not the case.
There is one known gene, and one unknown gene that both play a part in creating the abyssinian variety. It is an example of both mutations being required for the abyssinian trait to display. Both alter the expression of each other.
The first gene is Fzd6. On this gene there is a recessive mutation that "makes the skin cells face the opposite direction in the posterior of the mouse, eventually turning into whorls".
This mutation alone is not enough, and there is another unknown semi-dominant modifier that is also required for the rosettes or whorls to show on abyssinian. It has been given the placeholder code "M" for modifier.
Rst = Wild Type
rst = Rosette (Abyssinian) Component
M = Modifier
m = Wildtype
A mouse must be both homozygous for rosette (rst/rst) and have at least one copy of the modifier (M/*) to display as abyssinian. Since the modifier is semi-dominant, having 2 copies will produce more prominent whorls (rst/rst M/M) than just one copy.