A dominant phenotype can come from two different genotypes. A test cross pairs the individual with one that is homozygous recessive. Under a simple dominant–recessive model, a recessive offspring shows that the parent with the dominant phenotype carries a recessive allele.
The 2027 SEC G3 Biology K325 syllabus covers genetic diagrams for monohybrid inheritance. This guide compares the two possible crosses and shows why a small group of dominant offspring leaves some uncertainty.
- Choose one letter to represent the gene’s two alleles.
- List both possible genotypes for the dominant phenotype.
- Cross each possibility with the homozygous recessive genotype.
- Compare the predicted offspring with the observed result.
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Set up the genetic model
In a simplified example, T is an allele for tall plants and t is an allele for short plants. T is completely dominant over t, so both TT and Tt plants are tall. A tt plant is short.
A short tt plant contributes t in every gamete. That makes it easier to see which allele the tall parent passed on. A short offspring must receive t from both parents.
Keep genotype and phenotype separate in your answer. Tt is an allele combination. Tall is the observable trait in this model.
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Worked example | Two possible test crosses
A tall plant of unknown genotype is crossed with a short tt plant. Predict the offspring for each possible tall parent.
| Tall parent | Gametes from tall parent | Offspring genotypes | Expected phenotypes |
|---|---|---|---|
| TT | T only | All Tt | All tall |
| Tt | T or t | Half Tt and half tt | 1 tall : 1 short on average |
If one offspring is short, its genotype is tt. One t came from the short parent. The other t came from the tall parent, so that parent is Tt under the model.
In an exam genetic diagram, show the parental genotypes, the possible gametes and the resulting offspring genotypes before stating the phenotype ratio.
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Practise interpreting an all-tall group
Suppose a test cross produces four tall offspring. This result is consistent with a TT parent. It can also occur by chance with a Tt parent.
Under the Tt cross, each offspring has a one-half chance of being tall. If the outcomes are independent, the chance of four tall offspring in a row is (1/2)⁴ = 1/16.
The 1 to 1 ratio is an expectation over many offspring. A small group can depart from it, so describe what the observations support and whether they settle the genotype.
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Frequently asked questions
Why use a homozygous recessive parent?
It always passes on t in this example, so a recessive offspring reveals that the tall parent also passed on t.
Can two tall offspring have different genotypes in this test cross?
Each tall offspring must be Tt because the short parent always provides t.
Should I write 1 to 1 for every observed litter or seed group?
Use 1 to 1 for the expected phenotypic ratio in the Tt cross. State the actual counts separately when the question supplies observations.
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