Punnett Square Calculator — Genetics & Heredity Probability Tool

Punnett Square Calculator — Predict Genetic Inheritance

The Punnett Square Calculator is an essential genetics tool that helps you visualize and predict the probability of inheriting specific traits from two parents. Whether you're a student studying Mendelian genetics, a teacher explaining heredity, or just curious about how traits are passed down, our Punnett square maker gives you instant, accurate results.

What Is a Punnett Square?

A Punnett square is a diagram used to predict the genotype and phenotype combinations of offspring from two parents. It was developed by British geneticist Reginald Crundall Punnett in the early 20th century and remains the foundation of classical genetics education.

The square works by placing the gametes (sex cells) of each parent along the top and side of a grid. Each cell in the grid represents a possible offspring genotype, and the results show the statistical probabilities of each outcome.

How to Use the Punnett Square Calculator

1. Enter a trait name (optional) — for example, "Eye Color" or "Flower Color"
2. Choose a quick preset or type the genotype for Parent 1 (e.g., Aa, AA, or aa)
3. Enter the genotype for Parent 2 in the same format
4. Click "Build Punnett Square" to see the results

Genotype format:

• AA — Homozygous dominant (both alleles dominant)
• Aa — Heterozygous (one dominant, one recessive allele)
• aa — Homozygous recessive (both alleles recessive)

You can use any letter — for example, Bb, CC, or tt. The calculator automatically identifies the dominant allele (uppercase) and recessive allele (lowercase).

Punnett Square Examples

Example 1: Cystic Fibrosis (Carrier × Carrier)

Cystic fibrosis is an autosomal recessive genetic disorder. If both parents are carriers (Aa):

• 25% (1/4) — AA: Healthy, not a carrier
• 50% (2/4) — Aa: Healthy, carrier
• 25% (1/4) — aa: Affected with cystic fibrosis

Phenotypic ratio: 3:1 (dominant : recessive)

Example 2: Dominant × Recessive Cross (AA × aa)

When a homozygous dominant parent crosses with a homozygous recessive parent:

• 100% (4/4) — Aa: All offspring are heterozygous carriers
• All offspring show the dominant phenotype

Phenotypic ratio: 4:0 (all dominant)

Example 3: Carrier × Recessive (Aa × aa)

• 50% (2/4) — Aa: Carrier (dominant phenotype)
• 50% (2/4) — aa: Affected (recessive phenotype)

Phenotypic ratio: 1:1

Genotypic Ratio vs. Phenotypic Ratio

The calculator displays two types of ratios:

• Genotypic ratio — The proportion of each specific genotype combination (e.g., AA : Aa : aa = 1 : 2 : 1)
• Phenotypic ratio — The proportion of observable traits, where dominant phenotype includes both AA and Aa (e.g., dominant : recessive = 3 : 1)

Autosomal vs. Sex-Linked Inheritance

This calculator handles autosomal inheritance — traits encoded by genes on chromosomes 1–22 (not the sex chromosomes X and Y). Autosomal genes follow standard Mendelian rules:

• Each parent contributes one allele
• Dominant alleles mask recessive ones
• Offspring inherit one allele from each parent at random

Homozygous and Heterozygous Explained

• Homozygous dominant (AA) — Both alleles are dominant. The organism shows the dominant trait and cannot pass on the recessive allele.
• Heterozygous (Aa) — One dominant, one recessive allele. The organism shows the dominant trait but is a carrier of the recessive allele.
• Homozygous recessive (aa) — Both alleles are recessive. The organism shows the recessive trait.

Mendelian Inheritance

Our Punnett square is based on Gregor Mendel's laws of inheritance:

1. Law of Segregation — Each parent passes exactly one of their two alleles to each offspring.
2. Law of Independent Assortment — Alleles for different traits are inherited independently (this applies when genes are on different chromosomes).
3. Law of Dominance — One allele (dominant) can mask the expression of another (recessive) when both are present.

Frequently Asked Questions

What traits can I analyze with a Punnett square?

Any trait that follows simple Mendelian inheritance with two alleles — such as blood type (using just ABO allele pairs), earlobe attachment, tongue rolling, widow's peak, and many genetic disorders like cystic fibrosis, sickle cell anemia, and PKU.

What traits CANNOT be predicted with a Punnett square?

Traits controlled by multiple genes (polygenic), such as height, skin color, and intelligence, cannot be accurately predicted with a simple Punnett square because many loci are involved.

Can I use this for blood type inheritance?

Yes! You can analyze individual allele pairs. For example, to check if a child might receive the 'A' allele from a parent: enter the parent genotypes using 'A' (dominant) and 'a' (recessive) for each relevant allele pair.

What does "carrier" mean in genetics?

A carrier is an individual with a heterozygous genotype (Aa) for a recessive trait. They do not show the condition themselves but can pass the recessive allele to their children.