Construct monohybrid and dihybrid crosses
Predict inheritance using genetic diagrams for dominance, codominance, multiple alleles and sex linkage.
A Level; Cambridge International Biology 9700, 2028–2030. Written answers are self-checked; this is not practical-skills certification.
Before you begin
- Genes are DNA sequences that can be inherited; meiosis and fertilisation link parental genomes.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
When is a 9:3:3:1 dihybrid phenotype ratio expected?
What you will learn
- Construct gametes and Punnett squares for monohybrid and dihybrid crosses
- Interpret dominance, codominance, multiple alleles and sex-linked inheritance
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 |
Build the genetic explanation
Write parental genotypes, list their possible gametes and combine alleles in a Punnett square. For a complete-dominance monohybrid cross Aa×Aa, offspring genotypes are 1 AA:2 Aa:1 aa and the expected phenotype ratio is 3:1 under the stated assumptions. In codominance, both alleles contribute in a heterozygote; multiple-allele systems have more than two alleles in the population though each diploid individual carries at most two. Sex-linked diagrams place alleles on sex chromosomes and account for different inheritance patterns by sex. Dihybrid crosses track two loci; 9:3:3:1 requires independent assortment and complete dominance at both loci.
Check the evidence and limits
Predicted ratios depend on genotype, allele relationship, independent assortment and sample size. In sex-linked questions, state whether a probability denominator includes all offspring or only one sex. Do not use a 9:3:3:1 ratio for linked loci.
Pause and explain
A sex-linked recessive allele is on the X chromosome. Explain why a son can express it with one copy.
Worked example
For Aa×Aa with complete dominance, find the expected genotype ratio.
Show the worked solution
- Each parent produces A and a gametes.
- Combine the four equally likely fertilisation combinations.
Answer 1 AA : 2 Aa : 1 aa.
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 two heterozygous parents Aa×Aa. Give genotype and complete-dominance phenotype ratios.
Give me a hint
Set out the alleles or steps before deciding the outcome.
Compare my reasoning
- Gametes from each parent are A and a.
- The four combinations are AA, Aa, aA and aa.
Genotype ratio 1 AA:2 Aa:1 aa; phenotype ratio 3 dominant:1 recessive if A is completely dominant and the cross assumptions hold.
Look for these in your work
- Genetic terms used correctly
- Steps linked
In a codominant system, allele C1 and C2 are both expressed in a heterozygote. What phenotype is expected for C1C2?
Give me a hint
Use the given evidence and state your assumptions.
Compare my reasoning
- Do not mask either allele.
- Both allele products contribute.
The heterozygote shows both allele-associated features, because the alleles are codominant.
Look for these in your work
- Prediction or interpretation supported
- Limits stated
A sex-linked recessive allele is on the X chromosome. Explain why a son can express it with one copy.
Give me a hint
Apply the inheritance or gene-control model to the new context.
Compare my reasoning
- A son has one X chromosome and one Y.
- The Y may not carry a corresponding allele at that locus.
A son is hemizygous for many X-linked loci; if his single X carries the recessive allele and the Y lacks a corresponding allele, it can be expressed.
Look for these in your work
- Model applied accurately
- Unsupported certainty avoided
Common mistakes
- Skipping gametes and jumping directly to a ratio
- Using a dihybrid ratio when genes are linked
What can you explain now?
When is a 9:3:3:1 dihybrid phenotype ratio expected?
Compare with the explanation
With independent assortment and complete dominance at both loci
The ratio depends on those assumptions and equal gamete probabilities.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Which assumptions produce the standard monohybrid and dihybrid ratios?
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.