--- title: "Punnett Squares and Monohybrid Ratios" description: "Use a Punnett square to predict genotype and phenotype probabilities for a single-gene cross with complete dominance. Distinguish homozygous and heterozygous genotypes and explain variation in observe" canonical: https://lightmysky.com/learn/science/punnett-squares-and-monohybrid-ratios-mt_T4ZC45-77w source: https://lightmysky.com/learn/science/punnett-squares-and-monohybrid-ratios-mt_T4ZC45-77w.md retrieved: 2026-11-12 --- > **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`. # Punnett Squares and Monohybrid Ratios Use a Punnett square to predict genotype and phenotype probabilities for a single-gene cross with complete dominance. Distinguish homozygous and heterozygous genotypes and explain variation in observed offspring counts. Subject: Science · Area: Genetics & Evolution · Ages 15 to 16 Page: https://lightmysky.com/learn/science/punnett-squares-and-monohybrid-ratios-mt_T4ZC45-77w ## Ready when they can - Completes a Punnett square for a single-gene cross with complete dominance and states the expected phenotype ratio. - Uses homozygous for two identical alleles and heterozygous for two different alleles, without confusing genotype with phenotype. - Explains why a small offspring sample may not match the predicted ratio exactly. ## Lesson: Predicting offspring with a grid Mutations can create new alleles, which are versions of a gene. Here we follow one gene in diploid pea plants, with one allele inherited from each parent. Two identical alleles are homozygous: TT is homozygous dominant and tt is homozygous recessive. Two different alleles, Tt, are heterozygous. Genotype means the allele combination; phenotype means the observable characteristic. Use complete dominance here: T gives tall plants and t gives short plants. TT and Tt are tall; tt is short. A tall phenotype alone cannot distinguish TT from Tt. For seed shape, R gives round seeds and r gives wrinkled seeds: RR and Rr are round; rr is wrinkled. Other traits may follow different inheritance patterns. Gametes are reproductive cells. Each carries one allele for this gene. A heterozygous parent passes either allele with equal chance. To fill a Punnett square, write one parent's alleles across the top and the other's down the side. Combine one column allele with one row allele in every box. Write the capital letter first. Count genotypes, then group them by phenotype. **Example.** Cross Rr with Rr. Each box is equally likely. Gamete R r R RR Rr r Rr rr There are two routes to Rr. The genotype ratio is 1 RR to 2 Rr to 1 rr. RR and rr are homozygous, so half the boxes are homozygous. RR and Rr are round, giving 3 round to 1 wrinkled. **Example.** A homozygous parent has only one allele type to pass on. In RR crossed with rr, one parent always gives R and the other always gives r. Every offspring is Rr and has round seeds. Use the actual parental alleles for each cross. A different cross can give a different ratio. Each box gives a probability, not a fixed family count. In this model, fertilizations are independent: earlier offspring do not change the next one's chance. Expected count equals total offspring multiplied by probability. Rr by Rr gives an expected 200 x 1/4 = 50 wrinkled plants. Actual counts can differ. Larger samples tend to have proportions closer to predictions, but exact agreement is never required. *(drawing: The shaded part means a 1/4 chance of wrinkled seeds, not one guaranteed wrinkled plant in every group of four.)* **Recap.** Combine one allele from each parent in every box. Count genotypes, apply the stated dominance rule and treat the resulting ratios as probabilities. *Lesson revisions, questions and flashcards written for this dataset. OpenStax inheritance records were consulted for scientific facts and assessment design.* ## Practice 11 questions on this page, each with its working shown. ## Needs first - [Mutations and Their Effect on Proteins](https://lightmysky.com/learn/science/mutations-and-their-effect-on-proteins-mt_dTOQH3vGSz) - [Predicting Inherited Traits](https://lightmysky.com/learn/science/predicting-inherited-traits-mt_o7FJPDsHiW) ## Opens up - [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) - [The Hardy-Weinberg Principle and Allele Frequencies](https://lightmysky.com/learn/science/the-hardy-weinberg-principle-and-allele-frequencies-mt_eJjwZ2qVAE) - [Sex Determination and Inherited Disorders](https://lightmysky.com/learn/science/sex-determination-and-inherited-disorders-mt_PJQnvtnO0D)