Construct and interpret a Punnett square for a one-gene cross
A square combines gamete contributions before counting genotypes and phenotypes.
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Recall chromosomes, genes and alleles, meiosis, haploid gametes and fertilisation restoring two chromosome sets.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What belongs on a row or column label here?
What you will learn
- Combine one row allele and one column allele in each cell
- Read the cells as equally likely possibilities under the model
Original teaching model. Pedigrees and records are fictional; allele and probability assumptions are stated.
| Feature | Meaning |
|---|---|
| Column gametes | T and t from one parent |
| Row gametes | T and t from the other |
| Internal cells | TT, Tt, Tt, tt |
| Model interpretation | Four equal positions; three distinct genotype names |
Combine one row allele and one column allele in each cell
A Punnett square organises possible combinations of parental gamete alleles. For Tt × Tt, put T and t from one parent above the columns, and T and t from the other beside the rows. Each internal cell contains one row contribution and one column contribution. The four combinations are TT, Tt, Tt and tt, using the standard order for the same allele pair.
The labels outside the square describe gametes at the tracked gene; the internal pairs describe possible offspring genotypes. Do not put the full parental Tt record on each row or column. Keep an explicit allele key beside the model: T is the pigment-producing dominant allele and t is the recessive absence allele in these original examples.
Read the cells as equally likely possibilities under the model
Under equal gamete probabilities and independent combination, each of the four positions has probability one quarter. The repeated Tt genotype occupies two positions, so its probability is two quarters rather than one quarter. Distinct genotype names and equally likely positions are different counts: three genotype names occur among four positions.
Convert genotype to phenotype only after filling the square. TT and both Tt positions show pigment; tt does not, giving three pigment positions to one absence position. The square describes possible events and model probabilities, not four actual offspring that must occur together. If the parent genotypes or expression rule change, rebuild the combinations rather than copying the previous ratio.
Sources for this lesson
Pause and explain
Why does Tt have probability two quarters in Tt × Tt?
Worked example
Construct the four equal-probability positions for Tt × Tt.
Show the worked solution
- Place T and t above the columns and T and t beside the rows. Each entry represents one gamete contribution.
- Combine each row with each column: TT, Tt, Tt, tt. Count both Tt positions when computing probabilities.
Answer Rows T/t and columns T/t give TT, Tt, Tt, tt. Count Tt twice in the probability because it occupies two equal-probability positions.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Fill the square with rows T, t and columns T, t, recording the cells row by row.
Give me a hint
Join the row allele with the column allele.
Compare my reasoning
- Fill the T row as TT and Tt.
- Fill the t row as tT and tt.
- Write tT in standard order Tt.
The cells are TT, Tt, Tt and tt. They represent four possible equal-probability positions under the standard model.
Look for these in your work
- Four correct combinations
- tT and Tt recognised as the same genotype
Count the distinct genotype names and the positions belonging to each in that square. Give genotype probabilities.
Give me a hint
Do not confuse distinct names with repeated positions.
Compare my reasoning
- Count one TT, two Tt and one tt.
- Divide each count by four positions.
Three distinct genotypes: TT has 1/4, Tt has 2/4 or 1/2, and tt has 1/4.
Look for these in your work
- Correct 1:2:1 genotype position counts
- Correct probabilities with denominator four
Using T as dominant pigment, convert each square position to phenotype and correct the claim that four actual offspring must match the four cells exactly.
Give me a hint
Apply the expression key and distinguish possibilities from observations.
Compare my reasoning
- Assign pigment to TT and both Tt positions.
- Assign absence to tt.
- State the probabilistic interpretation.
Three positions show pigment and one shows absence, predicting a 3:1 phenotype ratio. Four actual offspring are not guaranteed to match the square exactly; its cells represent possible combinations and model probabilities.
Look for these in your work
- Correct 3:1 phenotype prediction
- Rejects guaranteed four-offspring matching
Common mistakes
- Putting two-allele parent genotypes in gamete labels
- Counting three distinct genotype names as three equally probable outcomes
- Reading four square cells as a guaranteed set of four offspring
What can you explain now?
Why does Tt have probability two quarters in Tt × Tt?
Compare with the explanation
It occurs in two equally likely square positions
T+t and t+T give the same genotype through two positions.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
Without your notes, combine one row allele and one column allele in each cell and read the cells as equally likely possibilities under the model. Use the supplied allele key and state what the evidence can establish.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.