| Epistasis Examples: Each Gene Pair Affecting the Same Character |
The next few pages give you a sampling of the possible types of interactions between two genes in a dihybrid cross (AaBb x AaBb). This is by no means a complete list. As complex as these interactions might seem, many phenotypes, such as coat color in horses, are the result of epistatic interactions among three or more genes. The elucidation of those patterns is quite difficult.
These examples are culled from a number of genetics books. The most important examples are from Monroe W. Strickberger (1985) Genetics. New York: MacMillan Publishing Co.,Third edition.
Punnett Square for the F2 of a dihybrid cross:
| AB | Ab | aB | ab | |
| AB | AABB | AABb | AaBB | AaBb |
| Ab | AABb | Aabb | AaBb | Aabb |
| aB | AaBB | AaBb | aaBb | aaBb |
| ab | AaBb | Aabb | aaBb | aabb |
Description 1: Complete dominance at both gene pairs; new phenotypes resulting from interaction between dominants, and also from interaction between both homozygous recessives.
Example: comb shape in chickens
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Allele Definitions: |
|
| Gene pair A: | rose (A) is dominant over nonrose (a) |
| Gene pair B: | pea (B) is dominant over nonpea (b) |
|
Interaction: |
|
| Dominants
for rose and pea (A_B_) produce walnut comb. Homozygous recessives for
rose and pea (aabb) produce single comb . |
|
| Genotype- Phenotype Relationships |
|
| A _ B_ | is walnut |
| A_ bb | is rose |
| aaB __ | is pea |
| aabb | is single |
Looking at the Punnett Square table you would expect walnut, rose, pea and single in a 9:3:3:1 ratio
| Phenotype |
Genotypes |
Ratios |
| walnut | AABB, AaBB, AABb, AaBb | 9/16 |
| rose | AAbb, Aabb | 3/16 |
| pea | aaBB, aaBb, | 3/16 |
| single | aabb | 1/16 |
Description 2: Complete dominance at both gene pairs, but one gene, when homozygous recessive, hides the phenotype of the other gene.
Example: mouse coat color
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Allele Definitions: |
|
| Gene pair A: | color dominant (A) over albino (a) (no pigment color) |
| Gene pair B: | agouti color (B) dominant over black (b) |
|
Interaction: |
|
|
homozygous albino (aa) hides the effect of agouti
(BB) and black (bb) |
|
|
Genotype- Phenotype Relationships |
|
| A _ B_ | agouti |
| A_ bb | black |
| aa_ _ | albino |
Looking at the Punnett square table you would expect agouti, black and white in a 9:3:4 ratio
| Phenotype |
Genotypes |
Ratios |
| agouti | AABB, AaBB, AABb, AaBb | 9/16 |
| black | AAbb, Aabb | 3/16 |
| albino | aaBB, aaBb, aabb | 4/16 |
Description 3: Complete dominance at both gene pairs, but either gene when homozygous recessive, hides the effect of the other gene.
Example: Flower color in sweet peas
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Allele Definitions: |
|
| Gene pair A: | purple (A) dominant over white (a) |
| Gene pair B: | color (B) dominant over colorless (b) (plants would be white) |
|
Interaction: |
|
|
homozygous white (aa)
or homozygous colorless (bb) produce the same phenotype |
|
|
Genotype- Phenotype Relationships |
|
| A _ B_ | purple |
| _ _ bb | white |
| aa_ _ | white |
Looking at the Punnett Square table you would have purple and white in a 9:7 ratio
| Phenotype | Genotypes | Ratios |
| purple | AABB, AaBB, AABb, AaBb | 9/16 |
| white | AAbb, Aabb, aaBB, aaBb, aabb | 7/16 |
Description 4: Complete dominance at both gene pairs, but one gene, when dominant, hides the phenotype of the other gene.
Example: sheep coat color
|
Allele Definitions: |
|
| Gene pair A: | White (A no pigment color) dominant over solid color (a) |
| Gene pair B: | black color (B) dominant over brown (b) |
|
Interaction: |
|
|
White (A _) hides the
effects of black (B_) or brown (bb) |
|
|
Genotype- Phenotype Relationships |
|
| A _ _ _ | white |
| aaB_ | black |
| aabb | brown |
Looking at the Punnett Square table you would have white, black and brown in a 12:3:1 ratio
| Phenotype | Genotypes | Ratios |
| white | AABB, AaBB, AABb, AaBb, AAbb, Aabb | 12/16 |
| black | aaBB, aaBb | 3/16 |
| brown | aabb | 1/16 |
| Strategy Page |
| Descriptions 1-4 |
| Descriptions 5-7 |
| Descriptions 8-10 |
| Sample Problems |