Punnett Square Calculator

Predict the genotypes and phenotypes of offspring from genetic crosses. Calculate probabilities for monohybrid and dihybrid crosses using Punnett squares.

Cross Type

Trait Information

Trait Name (e.g., "Flower Color")

Dominant Allele (e.g., "P")

Recessive Allele (e.g., "p")

Dominant Phenotype

Recessive Phenotype

Parent 1 Genotype

Parent 2 Genotype

Understanding Punnett Squares

A Punnett square is a diagram used to predict the genotypes and phenotypes of offspring from a genetic cross. Named after British geneticist Reginald Punnett, who devised the approach in 1905, it's a fundamental tool in classical genetics for visualizing Mendelian inheritance patterns.

Key Genetics Terms

Genotype

The genetic makeup of an organism - the actual alleles it possesses. Written as letter combinations (e.g., Pp, RRYy). The genotype determines what traits can potentially be expressed.

Phenotype

The observable characteristics or traits of an organism (e.g., purple flowers, brown eyes). The phenotype results from the expression of the genotype in combination with environmental factors.

Alleles

Different versions of the same gene. Represented by letters - uppercase for dominant alleles (e.g., P) and lowercase for recessive alleles (e.g., p). Each parent contributes one allele for each gene.

Dominant vs Recessive

Dominant alleles (uppercase) are expressed even when only one copy is present. Recessive alleles (lowercase) are only expressed when two copies are present (homozygous recessive). For example, in Pp, the dominant P is expressed.

Homozygous vs Heterozygous

Homozygous: Having two identical alleles (PP or pp). Heterozygous: Having two different alleles (Pp). Homozygous dominant (PP) and heterozygous (Pp) show the same phenotype, but different genotypes.

Mendel's Laws of Inheritance

Gregor Mendel's experiments with pea plants in the 1860s established the fundamental principles of heredity:

Law of Segregation

Each parent has two alleles for each gene, and these alleles segregate (separate) during gamete formation. Each gamete receives only one allele from each parent. When gametes combine during fertilization, offspring receive one allele from each parent, restoring the pair.

Law of Independent Assortment

Genes for different traits are inherited independently of each other (assuming they're on different chromosomes). This means the inheritance of one trait doesn't affect the inheritance of another. This is why dihybrid crosses (two traits) produce a 9:3:3:1 phenotypic ratio when both parents are heterozygous.

Law of Dominance

When two different alleles are present (heterozygous), one allele (dominant) will be expressed while the other (recessive) will be masked. The recessive trait only appears when two recessive alleles are present (homozygous recessive).

Classic Examples of Mendelian Traits

Mendel's Pea Plants

• Seed shape: Round (R) dominant over wrinkled (r)
• Seed color: Yellow (Y) dominant over green (y)
• Flower color: Purple (P) dominant over white (p)
• Pod shape: Inflated (I) dominant over constricted (i)
• Plant height: Tall (T) dominant over short (t)

Human Traits

• Widow's peak: Present (W) dominant over absent (w)
• Attached earlobes: Free (F) dominant over attached (f)
• Tongue rolling: Able (R) dominant over unable (r)
• Dimples: Present (D) dominant over absent (d)
• Freckles: Present (F) dominant over absent (f)

Note: Many human traits are actually polygenic (controlled by multiple genes) or show incomplete dominance, making real inheritance patterns more complex than simple Mendelian genetics.

Interpreting Punnett Square Results

Common Ratios:

Monohybrid Cross (Pp × Pp)

• Genotypic ratio: 1 PP : 2 Pp : 1 pp
• Phenotypic ratio: 3 Dominant : 1 Recessive
• Probability: 75% show dominant trait, 25% show recessive trait

Dihybrid Cross (RrYy × RrYy)

• Phenotypic ratio: 9:3:3:1
• 9/16 show both dominant traits
• 3/16 show first dominant, second recessive
• 3/16 show first recessive, second dominant
• 1/16 show both recessive traits

Test Cross (Pp × pp)

• Used to determine if an organism showing the dominant phenotype is homozygous or heterozygous
• If all offspring show dominant trait → parent is PP
• If 50% show each trait → parent is Pp

Limitations of Punnett Squares

While Punnett squares are excellent educational tools, they have limitations:

Predict probabilities, not certainties: A 3:1 ratio means 75% probability, but small sample sizes may not reflect this exactly
Assume simple dominance: Many traits show incomplete dominance, codominance, or polygenic inheritance
Don't account for linked genes: Genes on the same chromosome may be inherited together, violating independent assortment
Ignore environmental factors: Gene expression can be influenced by environment (epigenetics)
Limited to single or few genes: Most traits are polygenic (controlled by many genes)
Don't show mutations: New mutations occurring during gamete formation aren't represented
Assume equal gamete viability: Some gamete combinations may be lethal or less viable

References

The genetic principles and information used in this calculator are based on established genetics education resources:

Related Calculators

Genotype Ratio Calculator Phenotype Probability Calculator Allele Frequency Calculator Hardy-Weinberg Equilibrium Calculator Selection Coefficient Calculator

Educational Disclaimer: This Punnett square calculator is designed for educational purposes to demonstrate Mendelian genetics principles. Real genetic inheritance is often more complex, involving multiple genes, incomplete dominance, codominance, gene linkage, epistasis, and environmental factors. This tool assumes simple Mendelian inheritance with complete dominance and independent assortment. For actual genetic counseling or medical genetics questions, consult qualified genetics professionals.