Monohybrid inheritance and breeding outcomes
Use chromosome, gene and allele vocabulary to predict simple 1:1 and 3:1 inheritance outcomes and distinguish genotype from phenotype.
Section Paper 1 · Theory · subject content
Exam scope and assessment notes
Use this original lesson for the listed 2026, 2027–2028 and 2029–2031 Cambridge syllabus cycles. This route maps Cambridge's 2026 Version 2, 2027–2028 Version 1 and 2029–2031 Version 1 syllabuses. The 2026 update changes only a weblink; Cambridge reports no significant teaching changes for the 2027–2028 or 2029–2031 cycles. Exams are in the November series, and the qualification is not available in every administrative zone; confirm entry with your centre. Centre-selected local crop, weed, pest, disease and livestock examples must be checked against local agriculture. StudyForA does not replace supervised, risk-assessed practical work, the four assessed practical exercises, the individual practical investigation/report, teacher assessment or Cambridge moderation. Online practice is not an official mark or grade.
Before you begin
- Read the syllabus that matches your exam year and confirm your centre can enter you for code 0600. This route maps Cambridge's 2026 Version 2, 2027–2028 Version 1 and 2029–2031 Version 1 syllabuses. The 2026 update changes only a weblink; Cambridge reports no significant teaching changes for the 2027–2028 or 2029–2031 cycles. Exams are in the November series, and the qualification is not available in every administrative zone; confirm entry with your centre. Centre-selected local crop, weed, pest, disease and livestock examples must be checked against local agriculture. StudyForA does not replace supervised, risk-assessed practical work, the four assessed practical exercises, the individual practical investigation/report, teacher assessment or Cambridge moderation. Online practice is not an official mark or grade.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
For Aa × aa, what is the expected phenotype ratio if A is dominant?
What you will learn
- Define chromosome, gene, allele, homozygous, heterozygous, dominant and recessive.
- Calculate expected offspring ratios for simple monohybrid crosses.
- Distinguish genotype and phenotype and connect them to breeding decisions.
Use inheritance vocabulary accurately
Chromosomes carry genes; a gene is a unit of inherited information and an allele is a version of a gene. An organism with two matching alleles is homozygous; with two different alleles it is heterozygous.
In a simple model, a dominant allele affects phenotype when present; a recessive phenotype appears when no dominant allele is present. Genotype describes allele combination; phenotype is the observed characteristic.
Predict simple crosses
A Punnett square lists possible gametes and combines them. For two heterozygous parents Aa × Aa, expected genotypes are 1 AA : 2 Aa : 1 aa; if A is completely dominant, expected phenotypes are 3 dominant : 1 recessive.
A cross Aa × aa gives expected genotypes 1 Aa : 1 aa and a 1:1 phenotype ratio under the simple model. These are probabilities for offspring, not guarantees for a small family or litter.
Apply with limits
Breeding may select traits such as yield, disease resistance or hardiness, but a single-gene classroom model does not capture all real traits. Environment and multiple genes can affect phenotype.
Use the syllabus model for calculations and state its assumptions clearly.
Sources for this lesson
Pause and explain
For Aa × aa, what is the expected phenotype ratio if A is dominant?
Worked example
Two heterozygous parents Aa are crossed, with A dominant. Calculate the expected genotype and phenotype ratios.
Show the worked solution
- Each parent can pass A or a.
- The four combinations are AA, Aa, Aa and aa, giving a genotype ratio 1:2:1.
- Three combinations show the dominant phenotype and one shows the recessive phenotype, giving 3:1.
Answer The expected ratios are 1 AA : 2 Aa : 1 aa and 3 dominant : 1 recessive, assuming simple complete dominance.
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.
Cross Bb with bb and calculate the expected genotype ratio if B is dominant.
Give me a hint
List one allele from each parent for every possible combination.
Compare my reasoning
- Bb can produce B or b gametes; bb produces only b gametes.
- The combinations are Bb and bb in equal expected proportions.
- The expected genotype ratio is 1 Bb : 1 bb and phenotype ratio 1 dominant : 1 recessive.
The offspring are expected half Bb and half bb, giving a 1:1 phenotype ratio under the simple model.
Look for these in your work
- Gametes identified
- Genotype ratio correct
- Phenotype ratio correct
A breeder observes 28 dominant and 9 recessive offspring from a cross predicted to give 3:1. Explain how the results compare with the prediction.
Give me a hint
Compare the observed proportion with the expected ratio without demanding exact counts.
Compare my reasoning
- The total is 37; a 3:1 ratio predicts about 27.75 dominant and 9.25 recessive.
- The observed 28:9 is close to the expected 3:1 ratio.
- Small random variation is expected; the result does not prove the model for all traits.
The observed ratio is approximately 3:1 and is consistent with the simple prediction, allowing for random variation.
Look for these in your work
- Expected counts calculated
- Observed comparison accurate
- Probability caveat included
A farmer wants a visible trait such as coat colour to predict disease resistance. Explain why a monohybrid cross may be insufficient evidence.
Give me a hint
Consider genetic linkage, multiple genes, environment and validation.
Compare my reasoning
- The simple model predicts one gene with two alleles and a stated dominance relationship.
- Disease resistance may involve multiple genes, pathogen variation and environmental conditions.
- Use breeding records and local performance trials rather than inferring resistance from an unrelated visible trait.
A monohybrid calculation is a teaching model; it cannot establish a complex production trait without evidence linking genotype, phenotype and environment.
Look for these in your work
- Model assumptions named
- Complexity recognised
- Better evidence identified
Common mistakes
- Calling a dominant allele more common in a population
- Writing phenotype where genotype is requested
- Treating probability ratios as guaranteed counts
- Assuming one-gene inheritance describes every production trait
What can you explain now?
For Aa × aa, what is the expected phenotype ratio if A is dominant?
Compare with the explanation
1 dominant : 1 recessive
The cross produces Aa and aa in equal expected proportions.
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
Calculate Aa × Aa ratios and distinguish genotype from phenotype.
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.