#recessive 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

Duplicate recessive epistasis is also called

Duplicate recessive epistasis termed complementary gene interaction because phenotypic expression requires complementary presence of dominant alleles at two loci simultaneously to produce trait. Neither dominant alone can complete biochemical pathway; both must be present to generate end product like purple anthocyanin pigment in sweet pea or cyanogenic glucoside. Term complementary emphasizes cooperative interdependence where genes complement each other function. Contrasts with duplicate dominant where either locus sufficient, and recessive epistasis where one recessive masks. Complementary action yields diagnostic 9:7 ratio, hallmark of interdependent steps where loss at any step aborts pathway to same blocked phenotype.

Ref: Bateson & Punnett 1905, Sweet Pea; Griffiths Chapter 6: Complementary Gene Action Definition

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Gene intraction

Duplicate recessive epistasis produces which ratio?

Duplicate recessive epistasis, also called complementary gene action, occurs when homozygous recessive at either locus produces same alternate phenotype, requiring dominant alleles at both loci together for distinct phenotype. Cross AaBb x AaBb gives 9 A-B- showing novel phenotype needing both dominants, while 3 A-bb plus 3 aaB- plus 1 aabb all display same recessive alternative, totaling 7. Resulting ratio 9:7 signals two genes must function cooperatively to complete biosynthetic pathway, such as two enzymes acting sequentially. Neither single dominant alone sufficient, creating complementary requirement for pathway completion.

Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 6: Duplicate Recessive 9:7 Complementarity

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Gene intraction

An example of recessive epistasis is

Mouse coat color system involving C locus for tyrosinase pigment production and B locus for black versus brown exemplifies recessive epistasis. Genotype cc lacks tyrosinase, preventing melanin synthesis entirely, producing albino phenotype regardless of B- or bb genotype, thereby masking agouti brown variation. Meanwhile C-B- yields agouti, C-bb brown producing remaining visible classes. Combined albino class includes 3 ccB- plus 1 ccbb totaling 4, yielding 9:3:4 ratio. Sweet pea demonstrates 9:7 complementary, rice awn duplicate dominant, squash dominant epistasis, mouse illustrating supplementary recessive masking.

Ref: Cuénot and Castle, Early Mouse Genetics; Griffiths et al., Chapter 6: Mouse Coat as Recessive Epistasis Example

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Gene intraction

Recessive epistasis is also known as

Recessive epistasis often termed supplementary gene interaction because recessive homozygous epistatic genotype must act supplementarily with other gene to allow phenotype development. In this model, one gene supplements action of other, providing necessary substrate or cofactor; without supplement trait absent. Terminology supplementary distinguishes from complementary interaction where both dominants required simultaneously for novel trait, and duplicate interaction where either dominant suffices. Supplementary emphasizes recessive masking where homozygous recessive reveals epistasis. Historical literature uses supplementary to describe 9:3:4 pattern reflecting biochemical dependency between loci in pathway.

Ref: Strickberger, Genetics, 3rd ed., Chapter: Supplementary Gene Interaction Nomenclature

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Gene intraction

Recessive epistasis modifies the dihybrid ratio to

Recessive epistasis occurs when homozygous recessive genotype at epistatic locus aa masks expression of second locus, regardless of genotype at hypostatic locus. Phenotypically genotypes aaB- and aabb become indistinguishable because upstream block prevents downstream phenotype. Remaining segregating classes are 9 A-B- both dominant and 3 A-bb single dominant, plus combined 4 for masking class 3+1, producing ratio 9:3:4. Upstream gene may encode enzyme producing substrate required by downstream enzyme. Mouse albino cc preventing agouti and brown expression, and grain color maize are textbook illustrations.

Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 6: Recessive Epistasis and 9:3:4 Ratio

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