AP Syllabus focus:
‘Test crosses involve breeding an individual with a homozygous recessive partner to determine whether a dominant phenotype is homozygous or heterozygous.’
Test crosses are a classic genetics tool for inferring an organism’s genotype from its phenotype. They rely on predictable inheritance patterns when a dominant-phenotype individual is crossed with a recessive-phenotype partner.
Core idea and when to use it
A test cross is used when an individual shows a dominant phenotype but could have one of two genotypes. By choosing a mate with a known genotype, the offspring reveal which alleles the unknown parent contributes.
Test cross: A genetic cross between an individual with an unknown genotype (usually showing a dominant phenotype) and an individual that is homozygous recessive, used to determine the unknown genotype.
In AP Biology contexts, the “unknown” is typically written as A_ (could be AA or Aa), and the tester is aa.
Key requirement: a recessive tester
The tester must be homozygous recessive so that any dominant phenotype in offspring must come from the unknown parent.
Homozygous recessive: Having two copies of the recessive allele for a gene.
How the offspring outcomes determine genotype
This diagram summarizes the logic of a test cross by contrasting the two possible offspring patterns when a dominant-phenotype parent of unknown genotype is crossed to a homozygous recessive tester. If the unknown parent is homozygous dominant, all offspring are heterozygous and show the dominant phenotype; if the unknown parent is heterozygous, offspring segregate into a 1:1 ratio of dominant-phenotype heterozygotes and recessive homozygotes. The labeled Punnett squares make the allele contributions from each parent explicit, linking genotype to phenotype outcomes. Source
Because the homozygous recessive parent contributes only recessive alleles, the offspring’s phenotypes track the alleles coming from the unknown parent.
If the unknown parent is homozygous dominant (AA)
Cross: AA × aa
Gametes: unknown produces only A; tester produces only a
Offspring:
All Aa genotypes
All show the dominant phenotype
Interpretation: observing only dominant-phenotype offspring supports that the unknown parent is homozygous dominant, especially with a sufficiently large sample.
If the unknown parent is heterozygous (Aa)
Heterozygous: Having two different alleles for a gene.
Cross: Aa × aa
This Punnett square works through the classic heterozygous test cross, showing how a heterozygous parent produces two gamete types (A and a) while the recessive tester produces only a. The resulting offspring genotypes split evenly between and , producing a 1:1 dominant-to-recessive phenotype ratio under complete dominance. This visual makes clear why the appearance of any recessive-phenotype offspring reveals that the unknown parent carries the recessive allele. Source
Gametes: unknown produces A and a; tester produces only a
Offspring:
About half Aa (dominant phenotype)
About half aa (recessive phenotype)
Interpretation: the appearance of any recessive-phenotype offspring demonstrates the unknown parent carries the recessive allele and is therefore heterozygous.
Practical considerations in interpreting results
Sample size and chance
Real crosses produce finite numbers of offspring, so observed proportions can deviate from expected patterns by chance.
Small sample sizes can yield misleading “all dominant” outcomes even when the parent is heterozygous.
Larger offspring numbers increase confidence in the inference.
Controlling the cross
To make the inference valid:
Ensure the tester truly is homozygous recessive (often confirmed from its recessive phenotype for a simple dominance trait).
Prevent unintended fertilizations or pollinations that would mix genotypes.
Track offspring phenotypes carefully and use consistent criteria for scoring the trait.
What a test cross can and cannot tell you
A properly designed test cross can:
Distinguish AA from Aa when dominance is complete and the tester is aa.
A test cross may not give clear answers when:
The phenotype is difficult to score reliably (ambiguous categories).
Too few offspring are produced to detect rare recessive outcomes.
Environmental factors alter trait expression enough to obscure dominant vs recessive classification (a design/measurement issue rather than a different inheritance model).
FAQ
There is no fixed number; confidence rises with sample size.
In practice, aim for as many offspring as feasible, because a small brood can miss recessive phenotypes by chance even if the parent is heterozygous.
Treat “all dominant” as evidence, not absolute proof, of homozygous dominance.
Repeat the cross or increase offspring number to reduce the likelihood that chance alone hid recessive outcomes.
Yes, but scoring must be standardised.
Define clear phenotype categories before starting
Use blind scoring if possible
Consider quantitative measurements if the trait permits
You must prevent selfing to keep the tester parent as the only pollen/parental source.
Common approaches include removing anthers, bagging flowers, or using male-sterile lines, depending on the species.
A homozygous recessive tester contributes only recessive alleles, so offspring directly reveal which allele came from the unknown parent.
A heterozygous tester introduces dominant alleles that can mask what the unknown parent contributed, weakening the inference.
Practice Questions
An organism shows a dominant phenotype for a trait controlled by a single gene with complete dominance. Describe how a test cross can determine whether the organism is homozygous dominant or heterozygous. (2 marks)
States that the dominant-phenotype individual is crossed with a homozygous recessive individual (1)
Explains that all dominant offspring indicates homozygous dominant, whereas both dominant and recessive offspring indicates heterozygous (1)
A plant with a dominant phenotype is test crossed. The offspring include both dominant-phenotype and recessive-phenotype individuals. Explain, using alleles and gametes, why this result identifies the parent’s genotype, and state the expected pattern of offspring phenotypes. (5 marks)
Identifies the tester as homozygous recessive (aa) and that it produces only a gametes (1)
States the dominant-phenotype parent must be heterozygous (Aa) because recessive offspring occur (1)
Explains that the heterozygous parent produces two gamete types (A and a) (1)
Uses fertilisation logic to show Aa (dominant) and aa (recessive) offspring are formed (1)
States the expected approximate 1:1 dominant:recessive phenotype pattern (1)
