1. Epistasis

2. Definition
Epistasis is a form of gene interaction in which one pair of alleles (gene) masks the phenotypic expression of another.
Epistasis is a form of polygenics
The new phenotype is really the “absence” of the usual phenotype. It can result in albinism.

3. Epistatic versus Hypostatic
The alleles that are interferring or masking the effect are called epistatic alleles
The alleles whose effect is being interferred with or masked are called the hypostatic alleles.

4. Recessive or Dominant?
Epistasis can be described as either recessive epistasis or dominant epistasis.
Let’s look at an example of recessive epistasis….

5. Labs (the dogs!)
Fur color in Labs is controlled by two separate genes.
Fur color is a polygenic trait!
Gene 1: Represented by B
: Controls color
Gene 2: Represented by E
: Controls expression of B

6. Labs If a Lab has a dominant B allele, they will have black fur.
If they have two recessive alleles (bb) they will have brown fur.

7. Labs This would be considered “Recessive Epistasis”
If a lab receives at least one dominant “E” allele, they will remain the color that the “B” allele coded for.
Either black of brown
However, if a dog receives a pair of homozygous recessive “e” alleles, they will be golden regardless of their “B” alleles!
This would be considered “Recessive Epistasis”

8. Labs BBEE and BbEe --> Black lab bbEE and bbEe --> Brown lab
BBee, Bbee, or bbee --> Golden lab
Recessive Epistasis

9. Try this cross…
You have decided to cross your golden lab (bbee) with the neighbor’s black lab (BbEe). What color pups will they have? (Shortcut?)
Yup…..you have to do a dihybrid cross.

10. bbee x BbEe FOIL: be FOIL: BE, Be, bE or be
Genotypes of F1 generation: BbEe; Bbee; bbEe; bbee
Pups phenotypes: Black (25%); brown (25%)
and golden (50%)

11. Dominant Epistasis Let’s have a look at dominant epistasis…
Squash fruit color is controlled by two genes.
Gene 1 is represented by a W
Gene 2 is represented by a G

12. Squash Fruit Color Genotypes and Phenotypes: W_/G_ white W_/gg white
ww/G_ green
ww/gg yellow

13. Squash Fruit Color Which type of epistasis is represented here?
How do you know?
Dominant Epistasis
Because every time a dominant W allele
shows up in a squash genotype, the squash
fruit color is white.

14. Dominant Epistasis
Epistatic Alleles? WW
Hypostatic Alleles?
G alleles

15. Try this cross….
Cross a green squash (wwGg) with a white squash (Wwgg).
What color are the offspring?

16. Wwgg x wwGg FOIL: Wg or wg FOIL: wG or wg F1 generation genotypes:
Phenotypes: