Summary OCR A-Level Biology 6.2.8 Epistasis

Epistasis
Dominant Epistasis
 Presence of a dominant epistatic gene prevents the expression of a hypostatic gene,
regardless of its dominance
 Dominant epistatic gene in Labradors means that black pigment is produced which
overrides the pigment produced by the hypostatic gene, leading to black fur
 12:3:1 Example:
o A is a dominant epistatic gene coding for no pigment
o B is a hypostatic gene coding for yellow pigment
o b is a hypostatic gene coding for green pigment
Parent Phenotype Double Heterozygous x Double Heterozygous
Parent Genotype AaBb x AaBb
Parent Gametes AB Ab aB ab AB Ab aB ab
Dihybrid Cross AB Ab aB ab
A AAB AAB
AABB AaBb
B b b
A AAb
AABb AaBb Aabb
b b
AAB
aB AaBb aaBB aaBb
b
ab AaBb Aabb aaBb aabb

Ratio White = 12
Yellow = 3
Green = 1


 13:3 Example:
o A is a dominant recessive gene coding for no pigment
o B is a hypostatic gene coding for brown pigment
Parent Phenotype Double Heterozygous x Double Heterozygous
Parent Genotype AaBb x AaBb
Parent Gametes AB Ab aB ab AB Ab aB ab
Dihybrid Cross AB Ab aB ab
A AAB
AABB AaBb AABb
B b
Ab AABb AAbb AaBb Aabb
AAB
aB AaBb aaBB aaBb
b
ab AaBb Aabb aaBb aabb

Ratio White = 13