Punnett Square
Cross two parent genotypes to see the offspring genotype and phenotype ratios.
| B | b | |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
Phenotype ratio — 3 dominant : 1 recessive
A Punnett square crosses two parent genotypes to predict offspring ratios. For a Bb × Bb monohybrid cross, list each parent’s gametes (B and b) and fill four cells: 1 BB, 2 Bb, 1 bb. That’s a 1:2:1 genotype ratio and a 3:1 phenotype ratio — 75% dominant, 25% recessive.
How a Punnett square works
Each parent contributes exactly one allele per offspring, chosen at random from its two. A Punnett square lists each parent’s possible gametes along the edges and fills the cells with every combination. Counting the cells gives the expected offspring ratios. The genotype is the allele pair (BB, Bb, bb); the phenotype is the visible trait, and a single dominant allele is enough to show the dominant phenotype.
Worked example: Bb × Bb
Cross two heterozygous parents for a single gene:
- 1 List each parent’s gametes. Both parents are Bb, so each can pass on B or b.
- 2 Fill the four cells. B×B = BB, B×b = Bb, b×B = Bb, b×b = bb.
- 3 Count the genotypes. 1 BB : 2 Bb : 1 bb — a 1:2:1 genotype ratio.
- 4 Collapse to phenotypes. BB and Bb both look dominant; only bb looks recessive — the classic 3:1 phenotype ratio (75% dominant, 25% recessive).
Bb × Bb monohybrid cross outcomes
The classic Mendelian 1:2:1 genotype ratio and 3:1 phenotype ratio.
| Genotype | Count (of 4) | Probability | Phenotype |
|---|---|---|---|
| BB | 1 | 25% | Dominant |
| Bb | 2 | 50% | Dominant |
| bb | 1 | 25% | Recessive |
Reading the ratios, examples, and pitfalls
What the ratios mean. They are expected proportions over many offspring, not a guarantee. A 3:1 ratio means each offspring has a 75% chance of the dominant phenotype, but a single litter of four may not split exactly 3 and 1. To test whether observed counts match these expectations, run a chi-square goodness-of-fit test.
Real-world example. Crossing two pea plants heterozygous for seed shape (Rr, round dominant over wrinkled) gives about three round-seeded plants for every wrinkled one — the result Mendel famously observed. To scale these single-cross odds up to a whole population, see the Hardy-Weinberg calculator.
Common mistakes. Keep the case consistent — uppercase is the dominant allele, lowercase the recessive of the same gene. A heterozygote (Bb) is not a third “blended” phenotype under simple dominance; it looks identical to the homozygous dominant. And don’t confuse the 1:2:1 genotype ratio with the 3:1 phenotype ratio.