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Still 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 8: Complete dominance at one gene pair, partial dominance at the other; first gene, when homozygous recessive, is epistatic to second gene
Example: Hair direction in guniea pigs
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Allele Definitions: |
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| Gene pair A: | Rough (A) is dominant to smooth coat (a) |
| Gene pair B: |
Rough modifier (B) is incompletely dominant and modifies the expression of rough to smooth |
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Interaction: |
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Modifier acts only on rough hair (has no effect smooth
hair - aa) |
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Genotype- Phenotype Relationships |
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| A _ BB | smooth |
| A _ Bb | partly rough |
| A _ bb | completely rough |
| aa _ _ | smooth |
Looking at the Punnett Square table you would have smooth, partly rough, completely rough in a 7:6:3 ratio
| Phenotype | Genotypes | Ratios |
| smooth | AAbb, Aabb, aaBB, aaBb, aabb | 7/16 |
| partly rough | AABb, AaBb | 6/16 |
| completely rough | AABB, AaBB | 3/16 |
Description 9: Complete dominance at one gene pair, partial dominance at the other. Either gene when homozygous recessive hides the effects of the other gene. When both genes are homozygous recessive, the second gene hides the effects of the first gene.
Example: Body color in Tribolium castaneum (a flour beetle)
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Allele Definitions: |
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| Gene pair A: | red (A) is dominant to jet (a) |
| Gene pair B: | red (B) is partially dominant to black (b); the heterozygote is called sooty |
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Interaction: |
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When jet is homozygous recessive (aa) the effect of BB or Bb does not appear; the double recessive aabb is black |
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Genotype- Phenotype Relationships |
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| A _ BB | red |
| A_Bb | sooty |
| aaB_ | jet |
| A_bb & aabb | black |
| Phenotype | Genotypes | Ratios |
| red | AABB, AaBB, | 3/16 |
| sooty | AABb, AaBb | 6/16 |
| jet | aaBB, aaBb | 3/16 |
| black | aabb, AAbb, Aabb | 4/16 |
Description 10: Complete dominance for both gene pairs only if both kinds of dominant alleles are present. Otherwise the recessive phenotype shows up.
Example: Pigment glands in cotton plants
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Allele Definitions: |
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| Gene pair A: | Glands (A) are dominant to glandless (a) |
| Gene pair B: | Glands (B) are dominant to glandless (B) |
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Interaction: |
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| Dominants at A and B produce glands, homozygous dominant at A or B (AA or BB will produce glands even if the other gene is homozygous recessive). | |
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Genotype- Phenotype Relationships |
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| A _ B_ | glands |
| AAbb or aaBB | glands |
| Aabb or aaBb | glandless |
| aabb | glandless |
Looking at the Punnett square table you would have glands to glandless in an 11 to 5 ratio
| Phenotype | Genotypes | Ratios |
| glands | AABB, AABb, AAbb, AaBB, AaBb, aaBB | 11/16 |
| glandless | Aabb, aaBb, aabb | 5/16 |
| Strategy Page |
| Descriptions 1-4 |
| Descriptions 5-7 |
| Descriptions 8-10 |
| Sample Problems |