How to Solve a Monohybrid Cross with a Punnett Square

Monohybrid crosses are the foundation of classical genetics. They let you predict how a single gene with two alleles will be transmitted from parents to offspring. The most common tool for this prediction is the 2 × 2 Punnett square, a simple grid that combines the possible gametes of each parent.

What is a monohybrid cross?

A monohybrid cross examines the inheritance of one characteristic that is controlled by a single gene with two alternative forms (alleles). One allele is usually dominant (written with a capital letter) and the other recessive (written in lowercase). For example, the allele T (tall) is dominant over t (short) in pea plants.

When to use a Punnett square

Use a Punnett square whenever you need to determine the possible genotype (genetic makeup) and phenotype (observable trait) ratios of offspring from a known parental cross. It works for:

• Complete dominance (e.g., T > t)
• Simple heterozygous or homozygous parents

It is not suitable for polygenic traits, linked genes, or incomplete dominance without modification.

Step‑by‑step guide

1. Identify the parental genotypes. Write each parent’s genotype using a capital letter for the dominant allele and the same letter in lowercase for the recessive allele (e.g., Tt or tt) [1](https://examo.org/biology/genetics-and-inheritance/monohybrid-inheritance-and-punnett-squares).
2. List the possible gametes. Each parent can contribute one allele per gamete. A heterozygote (Tt) produces two types of gametes: T and t. A homozygous parent (TT or tt) produces only one type.
3. Draw a 2 × 2 grid. Place the gametes of one parent across the top and the gametes of the other parent down the side [2](https://pressbooks.umn.edu/classroompartners/chapter/punnett-squares/).
4. Fill in the squares. Combine the allele from the top with the allele from the side for each box. The result is the genotype of a possible offspring.
5. Determine phenotypes and ratios. Apply the dominant‑recessive rule to each genotype, count how many of each phenotype appear, and express them as a ratio (e.g., 3 : 1).

Worked example: Tall × Tall (Tt × Tt)

Suppose two pea plants are heterozygous for height (tall is dominant). Follow the steps:

1. Parental genotypes: Tt (father) × Tt (mother).
2. Possible gametes: each parent can produce T or t.
3. Draw the grid:
   

   	T	t
   T	TT	Tt
   t	Tt	tt

4. Genotype ratio: 1 TT : 2 Tt : 1 tt (1 : 2 : 1).
5. Phenotype ratio: TT and Tt are tall, tt is short → 3 tall : 1 short (3 : 1) [3](https://clockitwithsci.com/igcse/cambridge-igcse-biology/revision-notes/inheritance/monohybrid-inheritance/genetic-diagrams-and-pedigrees).

This result matches the classic Mendelian expectation for a heterozygous × heterozygous cross.

Common misconceptions

Self‑check

Question: Two pea plants, one homozygous tall (TT) and one heterozygous (Tt), are crossed. What are the genotype and phenotype ratios of the offspring?

Answer: The Punnett square gives genotypes TT and Tt in a 1 : 1 ratio, both phenotypically tall. Phenotype ratio = 2 tall : 0 short (or simply 100 % tall).

Practice variation

Try the same steps with a different gene, such as flower colour where purple (P) is dominant over white (p). Cross Pp × pp and calculate the expected ratios. This reinforces the method and shows how a homozygous recessive parent changes the outcome.