Use test crosses to infer genotype
Use offspring outcomes from a homozygous recessive cross to investigate an unknown dominant phenotype genotype.
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
Why is a homozygous recessive individual useful in a test cross?
What you will learn
- Explain the purpose of a test cross
- Interpret offspring phenotypes for monohybrid and dihybrid test crosses
Original teaching model. Pedigrees and records are fictional; allele and probability assumptions are stated.
| Feature | Meaning |
|---|---|
| Unknown pigment parent | TT or Tt under the supplied rule |
| Known partner | tt; supplies t only |
| Predicted TT case | Only Tt offspring; all possible outcomes show pigment |
| Predicted Tt case | Tt and tt equally; a tt record identifies Tt in the normal model |
Build the genetic explanation
An organism showing a dominant phenotype may be homozygous dominant or heterozygous. Cross it with an individual homozygous recessive for the gene being tested. If the unknown is homozygous dominant, all offspring are expected to show the dominant phenotype; if heterozygous, recessive-phenotype offspring are expected. A dihybrid test cross uses a double-recessive individual to reveal gamete combinations and possible linkage.
Check the evidence and limits
A small sample with no recessive offspring does not prove homozygosity; chance can produce a misleading sample. State expected outcomes and use sufficient offspring for stronger evidence.
Pause and explain
A double-heterozygous animal is test-crossed and parental combinations greatly exceed recombinant combinations. What does this suggest?
Worked example
A tall plant with unknown genotype is crossed with tt and some short offspring appear. Infer the unknown genotype.
Show the worked solution
- The recessive parent contributes t to every offspring.
- Short offspring must receive t from both parents.
Answer The unknown tall parent must carry t, so its genotype is Tt.
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.
An unknown dominant-phenotype individual crossed with aa produces recessive offspring. What does this show?
Give me a hint
Set out the alleles or steps before deciding the outcome.
Compare my reasoning
- The recessive offspring received a from both parents.
- The unknown parent therefore carried a.
The unknown parent is heterozygous Aa, because it contributed the recessive a allele to the recessive offspring.
Look for these in your work
- Genetic terms used correctly
- Steps linked
A test cross with a homozygous recessive plant yields 40 dominant offspring and no recessives. What is the strongest conclusion?
Give me a hint
Use the given evidence and state your assumptions.
Compare my reasoning
- The result is consistent with a homozygous dominant parent.
- A finite sample cannot prove this with certainty.
The result supports a homozygous dominant genotype, but a finite sample with no recessives cannot prove it absolutely; a heterozygous parent could produce such a sample by chance.
Look for these in your work
- Prediction or interpretation supported
- Limits stated
A double-heterozygous animal is test-crossed and parental combinations greatly exceed recombinant combinations. What does this suggest?
Give me a hint
Apply the inheritance or gene-control model to the new context.
Compare my reasoning
- A test cross reveals the gametes from the unknown parent.
- More parental than recombinant offspring is consistent with linkage.
It suggests the loci may be linked, because parental allele combinations are more frequent than recombinants. Crossing over can still produce some recombinant offspring.
Look for these in your work
- Model applied accurately
- Unsupported certainty avoided
Common mistakes
- Crossing the unknown with another dominant-phenotype organism
- Treating a small all-dominant sample as proof of homozygosity
What can you explain now?
Why is a homozygous recessive individual useful in a test cross?
Compare with the explanation
Its gametes all carry the recessive allele
Offspring phenotypes reveal which allele the unknown parent contributes.
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
How does a homozygous recessive test-cross partner reveal an unknown genotype?
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.