Separate predicted monohybrid ratios from observed offspring counts
Predictions, observations and numerical departures answer different questions.
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 denominator is used for 62 pigment-present records among 80 offspring?
What you will learn
- Calculate model expectations and observed percentages
- Use observations without treating a finite sample as proof of one cross
Original teaching model. Pedigrees and records are fictional; allele and probability assumptions are stated.
| Feature | Meaning |
|---|---|
| 3:1 expected total 80 | 60 present, 20 absent |
| Invented observed total 80 | 62 present, 18 absent |
| Observed percentages | 77.5% present; 22.5% absent |
| Departure | 2.5 percentage points; not proof of a causal mechanism |
Calculate model expectations and observed percentages
Predicted ratios come from the parental genotypes, gamete possibilities and expression rule. Under complete dominance, Tt × Tt predicts a 3:1 pigment phenotype ratio and Tt × tt predicts 1:1. These are probabilities under the stipulated normal model. An observed ratio comes from counting the actual supplied offspring records; it describes that sample rather than automatically establishing its parents.
For a predicted total of eighty from Tt × Tt, three quarters gives sixty pigment-present and one quarter gives twenty absent. An invented record of sixty-two present and eighteen absent still totals eighty. Its observed present percentage is sixty-two divided by eighty times one hundred, seventy-seven point five per cent; its absent percentage is twenty-two point five per cent.
Use observations without treating a finite sample as proof of one cross
The observed present percentage is two point five percentage points above the seventy-five per cent model expectation. This numerical departure is not by itself proof that the model is false, that a mutation occurred or that a particular parent has a different genotype. Chance sampling and the biological assumptions need to be considered; formal statistical testing is outside this lesson’s requirement.
Keep the denominator and wording clear. Expected sixty and twenty are not the recorded sixty-two and eighteen. The next offspring’s probability in the stipulated independent model does not change merely to force the final sample into a perfect ratio. If a task supplies uncertain parent genotypes, state the compatible possibilities and evidence limits rather than declaring certainty from a small observed ratio.
Sources for this lesson
Pause and explain
Does a predicted 3:1 ratio guarantee exact matching counts in every finite sample?
Worked example
Compare an 80-offspring 3:1 prediction with invented observed counts of 62 pigment-present and 18 absent.
Show the worked solution
- Multiply 80 by 3/4 and 1/4 to obtain expected counts 60 and 20.
- Divide the observations by 80 and multiply by 100: 77.5% and 22.5%. Pigment presence is 77.5 − 75 = 2.5 percentage points above the predicted fraction.
Answer Expected for 80 at 3:1: 60 present, 20 absent. Invented observations 62/18 give 77.5%/22.5%, a 2.5 percentage-point departure for pigment presence.
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.
Calculate expected present and absent counts for eighty offspring under a 3:1 phenotype prediction.
Give me a hint
Use three quarters and one quarter of the total.
Compare my reasoning
- Calculate 80 × 3/4.
- Calculate 80 × 1/4.
Expected present 60 and absent 20 under the model.
Look for these in your work
- Correct sixty and twenty
- Expected rather than observed wording
An invented sample has 62 present and 18 absent among 80. Calculate both percentages and the present percentage-point departure from 75%.
Give me a hint
Use eighty for both percentages.
Compare my reasoning
- Calculate 62/80 × 100.
- Calculate 18/80 × 100.
- Subtract 75 from the observed present percentage.
Present 77.5%, absent 22.5%; present is 2.5 percentage points above the 75% model expectation.
Look for these in your work
- Correct percentages
- Correct 2.5 percentage-point departure
A small sample from a known Tt × tt cross happens to show more pigment-present offspring. A student changes the next-offspring absence probability to compensate. Explain the error under the stated independent model.
Give me a hint
Previous observations do not impose a balancing rule.
Compare my reasoning
- Retain the known parental genotypes.
- Use one-half for each phenotype in the model.
- Distinguish finite observations from forced compensation.
The model still gives absence probability 1/2 for the next offspring. Earlier sample imbalance does not require compensation when the events are stipulated independent; a small observed departure does not change the known cross.
Look for these in your work
- Keeps one-half probability
- Rejects forced balancing from previous outcomes
Common mistakes
- Calling recorded counts the exact model prediction
- Using a departure alone to diagnose an inheritance mechanism
- Changing the next-event probability to force a perfect ratio
What can you explain now?
Does a predicted 3:1 ratio guarantee exact matching counts in every finite sample?
Compare with the explanation
No
It supplies probabilities under assumptions, while observed samples can differ.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Without your notes, calculate model expectations and observed percentages and use observations without treating a finite sample as proof of one cross. 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.