1. Recessive Epistasis
Complete dominance at both loci, but homozygous recessive condition at one of the two loci is epistatic to the other.
Flower Colour in blue-eyed Mary (Collinsia parviflora)
wwMM x
WWmm
White
Magenta
WwMm F1
Blue WM Wm wM wm
WWMM
WWMm
WwMM
WwMm
WWmm
Wwmm
wwMM
wwMm
wwmm F2
9 Blue : 3 Magenta : 4 White
Also Agouti : Black : Albino coat colour in mice

2. Enzyme 1 (w) is epistatic to Enzyme 2 (m)
Recessive Epistasis
Complete dominance at both loci, but homozygous recessive condition at one of the two loci is epistatic to the other.
Enzyme 1 (w) is epistatic to Enzyme 2 (m)

3. Duplicate Dominant Epistasis
Example: Seed capsule shape in Shepherd's purse (Capsella bursa-pastoris)
Plant 1 has a heart–shaped seed capsule and Plant 2 has a narrow capsule
Crossing the two produces an F1 with a heart-shaped capsule x
Single Dominant Gene?
Selfing the F1 produces the following F2 ratio: 15 heart to 1 narrow fruit

4. Duplicate Dominant Epistasis
Example: Seed capsule shape in Shepherd's purse (Capsella bursa-pastoris)
AABB x
aabb
Heart Shape
Narrow Shape
AaBb F1
Heart Shape AB Ab aB ab
AABB
AABb
AaBB
AaBb
AAbb
Aabb
aaBB
aaBb
aabb F2
A Dominant allele at either of the two loci produces a heart-shaped fruit
A is epistatic to B or b and B is epistatic to A or a to give a 15:1 ratio

5. Dominant Suppression Epistasis
Example: Malvidin production in Primula (anthocyanin giving blue flower)
Plants 1 and 2 lack blue pigment in their flowers.
When crossed together, the F1 also lacks pigment
Selfing the F1 produces blue flowered segregants
Phenotypic ratio: 13:3; Not 3:1

6. Dominant Suppression Epistasis
Example: Malvidin production in Primula (anthocyanin giving blue flower)
KKDD x
kkdd
Non blue
Non blue
KkDd F1
Non Blue KD Kd kD kd
KKDD
KKDd
KkDD
KkDd
KKdd
Kkdd
kkDD
kkDd
kkdd F2
Malvidin production controlled by dominant allele at K locus but the pathway is blocked by a dominant allele at the suppressor locus D
Produces a 13 : 3 ration, like feather colour in chickens

7. Bell Pepper Colour Genetics
prepared by Dr. Paul Kusolwa

8. Two genes without Epistasis - Additive
Example: Fruit colour in Bell Pepper
CCrr x
ccRR
Yellow
Brown
CcRr F1
Red CR Cr cR cr
CCRR
CCRr
CcRR
CcRr
CCrr
Ccrr
ccRR
ccRr
ccrr F2
A Dominant allele at either of the two loci produces red fruit
Dominant alleles at the C locus and no dominant alleles at R give yellow fruit
Dominant alleles at the R locus and no dominant alleles at C give brown fruit
Double recessive gives green fruit
9:3:3:1

9. Fruit shapes in squash Di locus Dominant to spherical or pyriform
Duplicate genes with cumulative effects
When Di is present together with Spherical S locus Di is dominant = Disc fruits
When Di present with recessive s = spherical fruits
When didi/ss = long or pyriform fruits
Modified Ratio = 9: 6: 1
9 Discs
Di_S_
1 Pyriform di/s
6 Spherical = Di/s & di/S_

10. Duplicate Recessive - Squash
Pathway involving two genes
Wt = warty fruits Dominant to non warty wt
Hr = hard rind
hr = intermediate texture
9:7 B_
Hr_
Wt_ Y_

11. More Epistasis in Squash
Bicolor fruits = locus B pleiotropic for fruits and leaves
For yellow and green color patches BB or Bb
Extent of yellow or green:
Model with 2 incompletely dominant additive loci, Ep1 and Ep 2, proposed for enlarging the yellow patches
Genotypes
Bb with a dosage of 0 to 1 Ep alleles = bicolor green and yellow fruits
Dosage of 2-4 dominant Ep alleles extends the yellow coloration
9 B_Ep_ Extended yellow
3 B_epep Yellow narrow
3 bbEp_ Green extended
1bbepep green

12. Tri-Genic Interactions….
Three or more genes interactions in ornamental gourds
Gb = green bands; gb for no bands
Gr/G = green rind (gr/g buff skin) L-2 = color intensity (yellow /orange) Gr Gb
L-2
Ep-1/2