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Examples of 2-locus epistasis
Recessive epistasis: Complete dominance at both loci, but homozygous recessive condition at one of the
two loci is epistatic to the other.
Example: coat color in mouse.
Locus 1: A color; a albino. Locus 2:B agouti; b black. Locus 1, when homozygous recessive, epistatic to
Locus 2.
.....................9..................... ..........3.......... .........3........ .....1.....
AABB AABb AaBB AaBb AAbb Aabb
aaBB aaBb
aabb
.................Agouti................. .......Black....... .............Albino...........
Phenotypic ratio: 9:3:4
Dominant epistasis: Presence of dominant allele at one locus always gives the same phenotype, regardless
of the allelic constitution at the other locus. Only when the dominant epistatic locus is homozygous
recessive can the allelic constitution of the second locus be expressed.
Example: fruit color in summer squash.
Locus 1: A white; a color. Locus 2: B yellow; b green.
.......................9...................... .........3.......... .........3........ ....1....
AABB AABb AaBB AaBb
AAbb Aabb
aaBB aaBb
aabb
...............................White............................... ....Yellow.... Green Phenotypic ratio: 12:3:1
Duplicate genes with cumulative effects: Complete dominance at both loci. Interaction between
dominants at both loci gives a new phenotype.
Example: fruit shape in summer squash.
Locus 1: A sphere; a long. Locus 2: B sphere; b long. A_B_ gives new phenotype:disc.
......................9....................... ..........3......... ........3......... ....1.... http://barleyworld.org/Plant_Gen/Lectures/Genes%20to%20phenos/2locepi.html[11/7/2011 3:17:01 PM]
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AABB AABb AaBB AaBb AAbb Aabb aaBB aaBb aabb ....................Disc.................... ................Sphere.................. Long
Phenotypic ratio: 9:6:1
Duplicate dominant genes: Dominant allele at each locus produces same effect, but not a new phenotype
when there are dominant alleles at both loci, as in the previous example.
Example: seed capsule shape in Shepherd's purse.
Locus 1: A triangular; a ovoid. Locus 2: B triangular; b ovoid.
.......................9......................
.........3......... ........3........ ...1...
AABB AABb AaBB AaBb
AAbb Aabb aaBB aaBb aabb
...........................................Triangular..................................... Ovoid
Phenotypic ratio: 15:1
Dominant and recessive interaction: Complete dominance at each locus. Locus 1, when dominant allele
present, is epistatic to locus 2. Locus 2, when homozygous recessive, is epistatic to Locus 1, if there is no
dominant allele present at Locus 1.
Example: feather color in the chicken.
Locus 1: A color inhibition; a color appearance. Locus 2: B color; b white. .......................9...................... ..........3......... .........3........ ....1....
AABB AABb AaBB AaBb
AAbb Aabb aaBB aaBb aabb
...............................White............................... .....Color..... White
Phenotypic ratio: 13:3
http://barleyworld.org/Plant_Gen/Lectures/Genes%20to%20phenos/2locepi.html[11/7/2011 3:17:01 PM]