--- title: "Dihybrid Crosses and the 9:3:3:1 Ratio" description: "Two gene pairs on different chromosomes assort independently, so a cross between two double heterozygotes gives four phenotypes in a 9:3:3:1 ratio. The grid is four by four and the ratio is a predicti" canonical: https://lightmysky.com/learn/science/dihybrid-crosses-and-the-9-3-3-1-ratio-mt_9IlVo04Dbt source: https://lightmysky.com/learn/science/dihybrid-crosses-and-the-9-3-3-1-ratio-mt_9IlVo04Dbt.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`. # Dihybrid Crosses and the 9:3:3:1 Ratio Two gene pairs on different chromosomes assort independently, so a cross between two double heterozygotes gives four phenotypes in a 9:3:3:1 ratio. The grid is four by four and the ratio is a prediction, not a law. Subject: Science · Area: Genetics & Evolution · Ages 17 to 18 Page: https://lightmysky.com/learn/science/dihybrid-crosses-and-the-9-3-3-1-ratio-mt_9IlVo04Dbt ## Ready when they can - Sets out the four gamete types from a doubly heterozygous parent. - Completes a four by four grid and reads the phenotype ratio off it. - Names the stage of meiosis at which independent assortment happens. ## Lesson: Crossing two genes at once A dihybrid cross tracks two genes at once, using parents heterozygous for both, such as RrYy crossed with RrYy. Two laws run the show. Segregation says the two alleles of one gene part into different gametes. Independent assortment adds that genes for different traits part independently, so each gamete gets a random mix of the two genes. A doubly heterozygous parent therefore makes four gamete types in equal numbers: RY, Ry, rY and ry, one allele of each gene per gamete. Learn that set cold, because every box of the grid is built from it. Dominant alleles mask recessive ones in any heterozygote, so uppercase letters win wherever they appear. **Example.** The four by four grid fills in mechanically and holds 16 boxes. List the four gamete types of each parent along the top and side, then combine them box by box. Count phenotypes, not genotypes: 9 boxes show both dominant traits, 3 show one mixed combo, 3 show the other, and 1 shows both recessives. That gives the predicted ratio 9:3:3:1. **Tip.** Pin the biology behind the numbers to metaphase I, when each homologous pair lines up facing one pole or the other and each pair chooses independently. That random lineup shuffles which alleles travel together. Two warnings: linked genes on one chromosome travel together and spoil the ratio, and the ratio is a prediction for real counts to wobble around, not a law. **Recap.** Four gamete types fill a 16 box grid, giving a predicted 9:3:3:1 phenotype split. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Gene Mutations and Their Consequences](https://lightmysky.com/learn/science/gene-mutations-and-their-consequences-mt_CDcaL6-iVp) - [Meiosis and Why Gametes Are Haploid](https://lightmysky.com/learn/science/meiosis-and-why-gametes-are-haploid-mt_hvu0bczcfX) - [Punnett Squares and Monohybrid Ratios](https://lightmysky.com/learn/science/punnett-squares-and-monohybrid-ratios-mt_T4ZC45-77w) ## Opens up - [Epistasis and Reading Pathway Order from Double Mutants](https://lightmysky.com/learn/science/epistasis-and-reading-pathway-order-from-double-mutants-mt_eE2WRPyYPk) - [Linkage, Recombination Frequency and Genetic Maps](https://lightmysky.com/learn/science/linkage-recombination-frequency-and-genetic-maps-mt_xBjZXt3TVn) - [Testing Genetic Ratios with Chi-squared](https://lightmysky.com/learn/science/testing-genetic-ratios-with-chi-squared-mt_YCO6jijrNQ)