How to Calculate Punnett Square Probabilities
A Punnett Square is a standard visual tool used in biology and genetics to predict the probability of an offspring inheriting particular traits. Invented by Reginald Punnett, this 2x2 grid computes a monohybrid cross, which examines the inheritance of a single trait governed by two alleles.
- Genotype: The genetic makeup of an organism, represented by two letters (e.g., Aa, AA, or aa).
- Dominant Alleles: Represented by capital letters (A). They mask recessive traits.
- Recessive Alleles: Represented by lowercase letters (a). They only show if the organism inherits two copies.
- Phenotype: The physical expression or observable trait resulting from the genotype.
| Parent Cross | Genotypic Ratio | Phenotypic Ratio |
|---|---|---|
| Aa x Aa (Heterozygous) | 25% AA : 50% Aa : 25% aa | 75% Dominant : 25% Recessive |
| AA x aa (Homozygous) | 100% Aa | 100% Dominant |
| Aa x aa | 50% Aa : 50% aa | 50% Dominant : 50% Recessive |
| AA x Aa | 50% AA : 50% Aa | 100% Dominant |
Frequently Asked Questions
What is the difference between homozygous and heterozygous?
An organism is homozygous if it inherits two identical alleles for a trait (like AA or aa). It is heterozygous if it inherits two different alleles (like Aa). Homozygous traits are purebred, whereas heterozygous traits are hybrid and will express the dominant characteristic.
Can a Punnett Square calculate multiple traits at once?
Yes. While this calculator performs a monohybrid cross (analyzing 1 trait via a 2x2 grid), you can also calculate two traits simultaneously using a dihybrid cross. Dihybrid crosses use a larger 4x4 grid (16 squares) and follow Mendel’s Law of Independent Assortment.
Why does the dominant trait hide the recessive trait?
In complete dominance, the protein coded by the dominant allele is sufficient to produce the trait's phenotype, effectively overshadowing the recessive allele. For the recessive phenotype to be physically expressed, the offspring must inherit two recessive alleles (aa) so no dominant protein is produced.