#dominant epistasis

Gene intraction

Dominant and recessive epistasis is also called

Interaction yielding 13:3 ratio characterized as dominant suppression where dominant allele at suppressor locus prevents expression of dominant allele at hypostatic pigment locus. Epistatic suppressor S- yields suppressed phenotype despite functional color allele B- present, so genotypes S-B- 9 plus S-bb 3 plus ssbb 1 all similar suppressed, totaling 13, leaving only ssB- 3 expressing color, giving 13 suppressed :3 colored. This dominant inhibitor acting epistatically over hypostatic gene classic in white leghorn chicken feathers, certain grains, demonstrates regulatory suppression mechanism rather than substrate limitation, distinguished from other epistatic ratios.

Ref: Strickberger, Genetics, 3rd ed., Chapter 9: Dominant Suppression and 13:3 Explanation

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Gene intraction

Dominant and recessive epistasis gives a phenotypic ratio of

Dominant and recessive epistasis combines dominant masking at one locus with recessive masking effect from interaction across loci, generating unusual 13:3 ratio. Genotypes containing dominant suppressor A- show suppressed phenotype regardless of B locus, while among aa genotypes, B- shows active phenotype and bb shares suppressed phenotype with A- class, causing grouping 9+3+1 =13 suppressed versus 3 active. Mechanistically dominant inhibitor in one pathway plus recessive loss blocks pigment, as seen in white feather color chicken and grain color. Pattern distinct from simple 12:3:1 dominant or 9:3:4 recessive epistasis.

Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 6: Dominant and Recessive Interaction 13:3

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Gene intraction

An example of duplicate dominant epistasis is

Rice awn character controlled by two loci where dominant allele at either locus promotes awn elongation via transcription factors activating cell elongation in parallel pathways. Biochemically either gene product suffices to trigger development. Cross heterozygous at both loci selfed produces 15 awned grains comprising 9 A-B- +3 A-bb +3 aaB-, and 1 awnless aabb, demonstrating diagnostic 15:1 duplicate dominant epistasis. This redundancy reflects selection during domestication where loss of one gene compensated by other. Pattern contrasts with squash dominant masking, sweet pea complementary both required, mouse recessive epistasis examples.

Ref: NCBI, Genetics of Rice Morphology; Griffiths et al., Chapter 6: Rice Awn Duplicate Dominant Illustration

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Gene intraction

Duplicate dominant epistasis occurs when

Duplicate dominant interaction implies functional equivalence between two loci where dominant allele at either locus complements loss at other locus because both control same biochemical step or parallel pathways. Presence of dominant genotype at first locus A- or second locus B- yields identical phenotype such as capsule dehiscence, awn formation, or pigment production. Only double recessive aabb lacks functional product, producing contrasting phenotype. Therefore dominant alleles interchangeable, explaining 15:1 ratio and protection against single-gene mutation, differing from complementary interaction requiring both dominants simultaneously for novel phenotype expression.

Ref: Strickberger, Genetics, 3rd ed., Chapter 9: Duplicate Genes and Functional Redundancy Mechanism

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Gene intraction

Dominant epistasis modifies the Mendelian ratio to

Dominant epistasis occurs when dominant allele at epistatic locus masks expression of alleles at hypostatic locus regardless whether hypostatic genotype dominant or recessive. Molecularly dominant inhibitor encodes repressor or diverts metabolite. Genotypes A-B- and A-bb both show epistatic masked phenotype because presence of A sufficient, remaining 3 aaB- shows hypostatic dominant phenotype, 1 aabb double recessive. Combining 9+3 yields first class 12, resulting in 12:3:1 ratio. Summer squash white fruit W masking yellow-green color exemplifies this pattern where one dominant allele prevents pigment development despite functional color genes.

Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 6: Dominant Epistasis Ratio 12:3:1

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