punnett square calculator

What is a Punnett square?

A Punnett square is a simple genetics tool that helps predict possible offspring genotypes from two parents. You place one parent’s possible gametes across the top of a grid and the other parent’s gametes down the side. Each box in the grid shows one possible genotype for a child.

This calculator automates that process and gives you:

• The full 2×2 Punnett square
• Genotype percentages (like AA, Aa, aa)
• Phenotype percentages (dominant vs recessive expression)
• Simplified Mendelian-style ratios

How to use this calculator

Step-by-step

• Enter the first parent’s genotype (two letters, such as Aa).
• Enter the second parent’s genotype (also two letters, such as aa).
• Optionally name the dominant and recessive phenotypes.
• Click Calculate Punnett Square.

Reading the results

Genotype frequencies

Genotypes are the actual allele combinations inherited by offspring. For a gene represented by A/a:

• AA: homozygous dominant
• Aa: heterozygous
• aa: homozygous recessive

The calculator reports both counts (out of 4 boxes) and percentages.

Phenotype frequencies

Phenotype is what is observed. In basic dominant/recessive inheritance:

• Any genotype with at least one uppercase allele (AA or Aa) shows the dominant phenotype.
• Only lowercase pairs (aa) show the recessive phenotype.

Classic example: Aa × Aa

A heterozygous cross (Aa × Aa) produces:

• Genotype ratio: 1 AA : 2 Aa : 1 aa
• Phenotype ratio: 3 dominant : 1 recessive

This is one of the most common examples in intro biology because it demonstrates Mendel’s law of segregation clearly.

When a Punnett square is most useful

• Learning or teaching basic Mendelian inheritance
• Quickly checking expected offspring distributions
• Visualizing genotype probabilities in monohybrid crosses
• Practicing for biology classes and exams

Limitations to keep in mind

Real genetics can be more complex than simple dominant/recessive patterns. A basic Punnett square does not capture:

• Incomplete dominance or codominance
• Polygenic traits (many genes involved)
• Epistasis and gene interactions
• Environmental effects on trait expression
• Linked genes and non-independent assortment

So while this calculator is excellent for foundational genetics, advanced inheritance models require deeper analysis.

FAQ

Can I enter lowercase first, like aA?

Yes. The calculator standardizes output format and still computes correctly.

Can I use letters other than A/a?

Yes. You can use any letter pair, like B/b or T/t, as long as both parents use the same gene letter.

Why do I get an input error?

Make sure each parent genotype is exactly two letters and represents one gene (for example, not “AB” or “Aaa”).

Final thoughts

A Punnett square calculator is one of the fastest ways to convert genotype notation into clear inheritance probabilities. Use it to learn, verify homework, build intuition, or quickly check likely offspring outcomes for single-gene crosses.