--- title: "Epistasis and Reading Pathway Order from Double Mutants" description: "When one gene masks the effect of another, the offspring ratios change in ways that say which gene acts first. Combining two mutations in one organism is how the order of steps in a pathway is worked " canonical: https://lightmysky.com/learn/science/epistasis-and-reading-pathway-order-from-double-mutants-mt_eE2WRPyYPk source: https://lightmysky.com/learn/science/epistasis-and-reading-pathway-order-from-double-mutants-mt_eE2WRPyYPk.md retrieved: 2026-09-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Epistasis and Reading Pathway Order from Double Mutants When one gene masks the effect of another, the offspring ratios change in ways that say which gene acts first. Combining two mutations in one organism is how the order of steps in a pathway is worked out. Subject: Science · Area: Genetics & Evolution · Ages 18 to 19 Page: https://lightmysky.com/learn/science/epistasis-and-reading-pathway-order-from-double-mutants-mt_eE2WRPyYPk ## Ready when they can - Reads a modified dihybrid ratio and names the interaction that produced it. - Infers which of two genes acts earlier in a pathway from the double mutant phenotype. - Separates epistasis from dominance rather than treating both as one gene overriding another. ## Lesson: When one gene masks another Epistasis happens when the genotype at one gene masks or changes the phenotype of a second gene. The dihybrid ratio then departs from 9 to 3 to 3 to 1 in a known way. A 9 to 7 split means both dominant alleles are needed. A 12 to 3 to 1 split means one dominant allele hides the other gene, and 9 to 3 to 4 means a homozygous recessive genotype blocks a later step. **Example.** Two white flowered plants are crossed and every child is purple. In the next generation purple and white appear in a 9 to 7 ratio. Each dominant allele alone gives white, and only both together give purple. That is complementary action. The order of steps in a pathway comes from double mutants. If the double mutant looks exactly like one single mutant, that gene acts later, because its block sits downstream. The earlier defect turns invisible once the later step is broken. This differs from dominance, which describes two alleles of one gene, not two genes. **Tip.** To read a strange ratio, first check the total fits sixteenths, which confirms two genes. Then group the classes into 9 to 7, 12 to 3 to 1, or 9 to 3 to 4. Linkage also distorts ratios, but only linkage answers a recombination analysis. **Recap.** Epistasis between two genes reshapes the dihybrid ratio, and the double mutant shows which gene acts later. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Dihybrid Crosses and the 9:3:3:1 Ratio](https://lightmysky.com/learn/science/dihybrid-crosses-and-the-9-3-3-1-ratio-mt_9IlVo04Dbt) - [Linkage, Recombination Frequency and Genetic Maps](https://lightmysky.com/learn/science/linkage-recombination-frequency-and-genetic-maps-mt_xBjZXt3TVn) ## Opens up - [Quantitative Traits, Variance Components and Heritability](https://lightmysky.com/learn/science/quantitative-traits-variance-components-and-heritability-mt_orJhIlYnKM)