# Epistasis Ratio Calculator (modified dihybrid ratios) Compute modified F2 phenotypic ratios from two-locus gene interaction: 9:7, 9:3:4, 12:3:1, 13:3, 15:1, 9:6:1 or 9:3:3:1, with expected counts and optional chi-square goodness-of-fit. Derived from Mendel 1866, Bateson & Punnett 1905, Griffiths, Hartl & Clark 2007. Educational use only. > Canonical page: https://elysiatools.com/en/tools/epistasis-ratio-calculator - **Category:** Education - **Keywords:** epistasis, epistasis ratio calculator, modified dihybrid ratio, 9:7 ratio, 9:3:4 ratio, 12:3:1 ratio, 13:3 ratio, 15:1 ratio, 9:6:1 ratio, gene interaction, complementary genes, mendelian genetics ## Overview The Epistasis Ratio Calculator computes expected F2 phenotypic ratios and counts for an AaBb × AaBb cross under independent assortment or six common two-locus interaction models: 9:7, 9:3:4, 12:3:1, 13:3, 15:1, and 9:6:1. Enter optional observed phenotype counts to perform a chi-square goodness-of-fit test. Educational use only. ## Inputs - **Interaction Type** (select): Type of gene interaction between the two loci. - **Total F2 offspring** (number): Total number of F2 individuals used to convert ratios into expected counts. - **Observed phenotype counts** (text): Optional comma-separated observed counts, one per phenotypic class, for a chi-square goodness-of-fit test. - **Decimal Places** (number) ## When to use - Calculate expected phenotype counts from a selected modified dihybrid ratio and total F2 offspring. - Compare observed phenotype counts with an independent 9:3:3:1 expectation or an epistasis ratio. - Review whether observed counts show a significant chi-square deviation from the selected theoretical ratio. ## How it works - Select the interaction type, such as complementary genes, recessive epistasis, or no interaction. - Enter the total number of F2 offspring to convert ratio parts into expected phenotype counts. - Optionally enter comma-separated observed counts in the same order as the phenotype classes. - The calculator returns JSON with the ratio, phenotype classes, expected counts, and optional chi-square goodness-of-fit results. ## Use cases - Teaching how two-locus gene interactions modify the classic Mendelian dihybrid ratio. - Estimating phenotype counts for classroom genetics exercises and F2 cross planning. - Testing example breeding data against a proposed epistasis model with chi-square goodness of fit. ## Frequently asked questions ### What cross does the calculator use? It models an AaBb × AaBb F2 cross with two loci. ### Which ratios are supported? It supports 9:3:3:1, 9:7, 9:3:4, 12:3:1, 13:3, 15:1, and 9:6:1. ### What should I enter for total F2 offspring? Enter the total number of F2 individuals used to calculate expected counts. ### How do I enter observed counts? Enter comma-separated counts, one for each phenotype class, following the listed class order. ### Is the calculator intended for clinical or research decisions? 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