Predict a three-to-one phenotype ratio from two heterozygous parents
Count allele pairs for genotype, then combine categories for phenotype.
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-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.
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
Which genotype shows the recessive phenotype here?
What you will learn
- Calculate genotype and phenotype ratios separately
- Convert model probabilities into expected counts
Original teaching model. Pedigrees and records are fictional; allele and probability assumptions are stated.
| Feature | Meaning |
|---|---|
| Square cells | TT, Tt, Tt, tt |
| Genotype ratio | TT:Tt:tt = 1:2:1 |
| Phenotype ratio | Pigment present:absent = 3:1 |
| Expected 80 | 60 present, 20 absent; observations need not match exactly |
Calculate genotype and phenotype ratios separately
In the complete-dominance pigment model, Tt × Tt gives TT, Tt, Tt and tt in the equal-position square. The genotype counts are one TT, two Tt and one tt, written 1:2:1. Genotype ratios count allele combinations. Do not use that three-category ratio when the question asks for the two observable phenotype categories.
TT and Tt both express the dominant pigment phenotype, while tt expresses absence. Combining those equal positions gives three pigment to one absence, a 3:1 phenotypic ratio. The corresponding probabilities are three quarters and one quarter, or seventy-five per cent and twenty-five per cent. The two Tt positions belong with TT when grouping by phenotype.
Convert model probabilities into expected counts
To calculate expected counts, multiply the total by each model probability. For eighty offspring under this model, the expected phenotype counts are sixty with pigment and twenty without. The expected genotype counts are twenty TT, forty Tt and twenty tt. Both sets add to eighty, but they organise the outcomes into different categories.
Expected counts are predictions under equal transmission, independent combination and the stated expression rule. An observed sample can differ through chance and other circumstances that need evidence. A 3:1 prediction is not a guarantee that every consecutive group of four offspring contains exactly three pigment-producing individuals. Report expected when calculating from the model, and observed when counting supplied records.
Sources for this lesson
Pause and explain
What is the predicted pigment-present to absent phenotype ratio for Tt × Tt?
Worked example
For Tt × Tt, calculate expected genotype and pigment counts in a total of 80 offspring.
Show the worked solution
- The four positions are TT, Tt, Tt and tt: genotype fractions 1/4, 1/2 and 1/4. Multiply by 80 to obtain 20, 40 and 20.
- Group TT and Tt as pigment present under the T dominance rule. The expected phenotype counts are 60 present and 20 absent, giving 3:1.
Answer For an expected total of 80 from Tt × Tt: genotypes 20 TT, 40 Tt, 20 tt; phenotypes 60 pigment, 20 absent. Genotype 1:2:1 and phenotype 3:1 are different groupings.
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.
From TT, Tt, Tt, tt, give the genotype ratio and the pigment phenotype ratio with category labels.
Give me a hint
Group once by alleles, then by observable expression.
Compare my reasoning
- Count TT:Tt:tt as one:two:one.
- Group TT and Tt for pigment.
Genotypes TT:Tt:tt = 1:2:1. Phenotypes pigment present:absent = 3:1 under the supplied dominance rule.
Look for these in your work
- Correct labelled genotype ratio
- Correct labelled phenotype ratio
For 120 offspring predicted by the same model, calculate expected pigment-present and absent counts.
Give me a hint
Multiply the total by three quarters and one quarter.
Compare my reasoning
- Calculate 120 × 3/4.
- Calculate 120 × 1/4.
- Check the two expected counts sum to 120.
Expected pigment present 90 and absent 30. These are model expectations, not guaranteed observed counts.
Look for these in your work
- Correct 90 and 30
- Labels counts expected
A student says the next four offspring must include exactly one without pigment because the predicted ratio is 3:1. Correct this interpretation.
Give me a hint
A square gives probabilities for possible combinations.
Compare my reasoning
- Identify the one-quarter absence probability.
- Distinguish it from a compulsory four-outcome sequence.
The model gives absence probability 1/4 for each offspring under its assumptions. It does not guarantee exactly one absence in the next four; observed small samples can differ from the expected ratio.
Look for these in your work
- States one-quarter probability
- Rejects guaranteed small-group composition
Common mistakes
- Calling 1:2:1 the phenotype ratio under complete dominance
- Excluding heterozygotes from the dominant phenotype count
- Reporting expected counts as guaranteed observations
What can you explain now?
What is the predicted pigment-present to absent phenotype ratio for Tt × Tt?
Compare with the explanation
3:1
Three equal square positions have T and one is tt.
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, calculate genotype and phenotype ratios separately and convert model probabilities into expected counts. 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.