Punnett Square and Genetics
Predict offspring genotypes and phenotypes using Punnett squares.
Covers dominant, recessive, and codominant traits.
The Formula
Monohybrid Cross (Aa × Aa):
Genotype ratio: 1 AA : 2 Aa : 1 aa
Phenotype ratio: 3 dominant : 1 recessive
Probability of genotype = (favorable outcomes) / (total outcomes)
Genotype ratio: 1 AA : 2 Aa : 1 aa
Phenotype ratio: 3 dominant : 1 recessive
Probability of genotype = (favorable outcomes) / (total outcomes)
A Punnett square is a grid that predicts the possible genotypes of offspring from two parents. Each parent contributes one allele, and the grid shows all possible combinations.
Variables
| Term | Meaning |
|---|---|
| Allele | A version of a gene (e.g., A or a) |
| Genotype | The genetic makeup (e.g., AA, Aa, aa) |
| Phenotype | The observable trait (e.g., tall, short) |
| Dominant (A) | Allele that shows its effect when one or two copies are present |
| Recessive (a) | Allele that only shows its effect when two copies are present |
| Heterozygous | Two different alleles (Aa) |
| Homozygous | Two identical alleles (AA or aa) |
Example 1
Cross two heterozygous parents (Aa × Aa)
Punnett square: AA, Aa, Aa, aa
Genotype ratio: 1 AA : 2 Aa : 1 aa
75% dominant phenotype, 25% recessive phenotype
Example 2
Cross a heterozygous (Aa) with a homozygous recessive (aa)
Punnett square: Aa, Aa, aa, aa
Genotype ratio: 2 Aa : 2 aa
50% dominant phenotype, 50% recessive phenotype
When to Use It
Use Punnett squares when:
- Predicting the probability of genetic traits in offspring
- Determining carrier status for genetic conditions
- Planning breeding programs in agriculture or animal husbandry
- Understanding patterns of inheritance in biology courses