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Some more Epistasis examples: Each Gene Pair Affecting the Same Character |
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 5: Complete dominance at both gene pairs, but either gene, when dominant hides the effects of the other gene.
Example: Seed capsules of Shepard's purse
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Allele Definitions: |
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| Gene pair A: | triangular (A) is dominant to ovoid (a) |
| Gene pair B: | triangular (B) is dominant to ovoid (b) |
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Interaction: |
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Dominant allele at
either the A or B locus hides the ovoid phenotype |
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Genotype- Phenotype Relationships |
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| A _ _ _ | triangular |
| _ _ B_ | triangular |
| aabb | iovoid |
Looking at the Punnett Square table, the triangular: ovoid phenotypes would occur in a 15:1 ratio
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Phenotype |
Genotypes |
Ratios |
| triangular | AABB, AaBB, AABb, AaBb, AAbb, Aabb, aaBB, aaBb, | 15/16 |
| ovoid | aabb | 1/16 |
Description 6: Complete dominance at both gene pairs. One gene, when dominant, hides the effects of the second gene. The second gene, when homozygous recessive, hides the effects of the first gene.
Example: Feather color in chickens
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Allele Definitions: |
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| Gene pair A: | color inhibition (A) is dominant to color presence (a) |
| Gene pair B: | color (B) dominant to white (b) |
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Interaction: |
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dominant color
inhibition (Aa)
or homozygous white (bb) produce the same phenotype |
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Genotype- Phenotype Relationships |
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| A _ _ _ | white |
| _ _ bb | white |
| aaB _ | colored |
Looking at the Punnett square table, the white:colored phenotypes would occur in a 13:3 ratio
| Phenotype | Genotypes | Ratios |
| white | AABB, AaBB, AABb, AaBb, AAbb, Aabb, aabb | 13/16 |
| colored | aaBB, aaBb | 3/16 |
Description 7: Complete dominance for both gene pairs, interactions between both dominants to give a new phenotype.
Example: Fruit shape in summer squash
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Allele Definitions: |
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| Gene pair A: | Sphere (A) is dominant to long (a) |
| Gene pair B: | Sphere (B) is dominant to long (b) |
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Interaction: |
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Dominant sphere
(A) interacts with dominant sphere (B) to give disc.
Either dominant sphere allele (A or B) hides the
effects of long. |
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Genotype- Phenotype Relationships |
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| A _ B_ | disc |
| A_bb or aaB_ | sphere |
| aabb | long |
Looking at the Punnett square table, the disc, sphere and long phenotypes would occur in a 9:6:1 ratio
| Phenotype | Genotypes | Ratios |
| disc | AABB, AaBB, AABb, AaBb, | 9/16 |
| black | aaBB, aaBb, AAbb, Aabb | 6/16 |
| brown | aabb | 1/16 |
Look at still more example of epistasis
| Strategy Page |
| Descriptions 1-4 |
| Descriptions 5-7 |
| Descriptions 8-10 |
| Sample Problems |