Punnett Square and Genetics
Predict offspring genotypes and phenotypes using Punnett squares for dominant, recessive, codominant, and X-linked traits with probability percentages.
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
Key Notes
- Structure: A Punnett square is a grid where each row represents one parent's alleles and each column represents the other's. Each cell shows one possible offspring genotype. A monohybrid cross (one gene) uses a 2×2 grid; a dihybrid cross (two genes) uses 4×4.
- Dominant vs recessive alleles: Uppercase letters (A) denote dominant alleles; lowercase (a) denote recessive. A single dominant allele (Aa or AA) produces the dominant phenotype; two recessive alleles (aa) are needed to show the recessive trait.
- Classic Mendelian ratios: Two heterozygous parents (Aa × Aa) give AA : Aa : aa = 1 : 2 : 1 (genotype) and dominant : recessive = 3 : 1 (phenotype). A dihybrid cross (AaBb × AaBb) gives a 9 : 3 : 3 : 1 phenotype ratio.
- Independent assortment assumption: Punnett square predictions assume genes on different chromosomes assort independently (Mendel's second law). Genes that are physically close on the same chromosome (linked) deviate from these predicted ratios.
- Applications: Punnett squares are used in genetic counseling to estimate the probability of children inheriting a recessive disease, in animal and plant breeding programs, and to analyze inheritance patterns of dominant diseases like Huntington's.