---
title: "Punnett Squares and Monohybrid Ratios"
description: "Draw a Punnett square for a cross between two known genotypes and read off the expected ratio of offspring. Covers dominant and recessive alleles, homozygous and heterozygous, and genotype against phe"
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: 2026-09-12
---

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# Punnett Squares and Monohybrid Ratios

Draw a Punnett square for a cross between two known genotypes and read off the expected ratio of offspring. Covers dominant and recessive alleles, homozygous and heterozygous, and genotype against phenotype.

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 monohybrid cross and states the phenotype ratio.
- Uses the right term for a genotype with two identical alleles and for one with two different alleles.
- Explains why a predicted 3 to 1 ratio rarely comes out exactly in a real litter.

## Lesson: Predicting offspring with a grid

Traits come in versions called alleles. Each parent gives one allele per gene. A dominant allele masks a recessive partner. Two identical alleles make a homozygous genotype such as TT or tt. Two different alleles make a heterozygous genotype such as Tt. The genotype is the letters, the phenotype is what you see.

**Example.** Cross two heterozygous tall pea plants, Tt with Tt. Put T and t across the top and side of a two by two grid, then fill the four boxes: TT, Tt, Tt, tt. Three boxes show the tall phenotype and one shows short, so the expected ratio is 3 tall to 1 short.

That ratio is a probability, not a promise. Each birth rolls the same dice on its own, so a litter of four can easily miss 3 to 1. Mendel trusted big counts because patterns settle near predictions only in large numbers.

**Tip.** Work every cross the same way. Write the parental alleles on the grid edges, fill each box, then count genotypes before reading off phenotypes. Name each genotype with its exact term as you go.

**Recap.** Grid the alleles, count the boxes, read the ratio, and trust it only in large numbers.

## 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)