Punnett Squares: Genotype and Phenotype Ratios Explained
Fill in a Punnett square, read genotype and phenotype ratios, and avoid the mistake of thinking offspring must match the ratio exactly.
The vocabulary
- Genotype · the letters, like Tt
- Phenotype · the visible trait
- Dominant · shown with one copy (T)
- Recessive · needs two copies (tt)
Genotype is the code. Phenotype is what you see.
How to fill it in
- Put one parent's alleles across the top
- Put the other parent's down the side
- Combine each row with each column
Each of the four boxes is an equally likely offspring.
Example 1
Tt × Tt. What fraction is tt?
- The four boxes are TT, Tt, Tt and tt
- One box out of four is tt
1/4, or 25%
Example 2
Bb × bb. What is the genotype ratio?
- The four boxes are Bb, Bb, bb and bb
- Two Bb and two bb
1 Bb : 1 bb
Common mistake
Four kids from Tt × Tt must include exactly one tt
Each child has a 1 in 4 chance, independent of the others.
The ratio is a probability, not a guarantee.
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