15 public questions tagged with this topic.
Epistasis occurs when one gene suppresses the expression of another, as seen in albinism.
Ref: NCERT Class 12 Biology Chapter 4: Principles of Inheritance and Variation Non-Mendelian Inheritance - Incomplete Dominance Co-dominance Multiple Alleles
Epistasis occurs when one gene masks or modifies the expression of another gene.
Ref: NCERT Class 12 Biology Chapter 4: Principles of Inheritance and Variation Mendel's Laws - Dominance Segregation Independent Assortment
Epistasis describes functional masking where genotype at one locus alters phenotypic expression of another non-allelic locus without changing meiotic segregation behaviour itself. Monohybrid 3:1 ratios persist because alleles still separate at anaphase, yet combinational phenotypes collapse when biochemical steps interact in same pathway. Dihybrid expectation 9:3:3:1 therefore becomes modified into variants like dominant 12:3:1, complementary 9:7, duplicate 15:1 or recessive 9:3:4 depending on whether inhibitor, redundant or cooperative enzymes are involved. Hence alteration of dihybrid ratio serves as diagnostic signature of epistatic interaction.
Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 9: Epistasis Modified Dihybrid Ratios
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
Sweet pea Lathyrus odoratus flower color requires dominant alleles at two loci C and P encoding enzymes catalyzing anthocyanin biosynthesis. C- converts colorless precursor to intermediate, P- converts intermediate to purple pigment; absence of either dominant halts pathway yielding white flowers. Cross CcPp x CcPp selfed produces 9 C-P- purple and 7 white comprising 3 C-pp plus 3 ccP- plus 1 ccpp. Ratio 9:7 demonstrates complementary interaction where both dominants needed. Bateson, Punnett, and Saunders classic 1905 experiment discovering this founded concept of gene interaction and duplicate recessive epistasis.
Ref: Bateson, Punnett, Saunders 1905, Sweet Pea Experiment; Griffiths 12th ed., Chapter 6: Complementary 9:7
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
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
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
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
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
In epistatic interaction terminology, epistatic locus exerts masking effect preventing second locus from manifesting its genotype phenotypically, while hypostatic locus refers to locus whose expression suppressed. For example, dominant white W epistatic to color genes in squash, recessive cc epistatic to agouti extension in mice. Hypostatic gene may carry fully functional alleles, but phenotypic impact invisible because biochemical pathway blocked upstream or inhibited downstream. Clarifying which locus epistatic versus hypostatic helps order genes in pathway, distinguishing upstream enzyme from downstream modifier or regulatory step.
Ref: Snustad & Simmons, Principles of Genetics, 7th ed., Chapter 4: Epistatic and Hypostatic Definitions
Epistasis represents inter-locus interaction where genotype at one locus masks or modifies phenotypic expression of genotype at second non-allelic locus. Underlying mechanism often involves genes encoding sequential enzymes in same biosynthetic pathway where upstream mutation blocks substrate production, preventing downstream gene phenotype regardless of its genotype, or regulatory factor suppresses transcription of target. Outcome changes dihybrid phenotypic expectation from 9:3:3:1 to modified ratios like 12:3:1 or 9:3:4. Concept distinct from dominance which is intra-locus allele interaction and from linkage concerning physical chromosome proximity.
Ref: Bateson, 1909, Term Epistasis; Griffiths et al., Chapter 6: Mechanisms of Epistatic Masking
