Test cross: finding the genotype
You cannot tell from a dominant phenotype whether an individual is AA or Aa. Cross it with a homozygous recessive partner (aa) and the offspring reveal the genotype.
Rules
- Test cross: unknown × aa
- All offspring dominant → AA
- About half recessive (1 : 1) → Aa
- The aa partner only passes on a, so the phenotype shows the tested parent’s allele directly
Your turn
Three problems on this topic. Type probabilities as a percentage number, e.g. 18.75.
Task
Trait: seed colour in peas, A dominant over a. An individual with the yellow phenotype is crossed with a homozygous recessive partner (aa). There are 61 offspring with the yellow and 69 with the green phenotype. What was the genotype of the tested parent, and what percentage of the offspring is recessive?
Genotype of the tested parent
Share of recessive offspring in %
Probabilities as a percentage number (25 or 18.75), rounded to 2 decimal places; allele frequencies as a decimal (0.02).
Worked example
Trait: flower colour in peas, A dominant over a. An individual with the purple phenotype is crossed with a homozygous recessive partner (aa). There are 48 offspring with the purple and 46 with the white phenotype. What was the genotype of the tested parent, and what percentage of the offspring is recessive?
Worked solution
- 1.Test cross with aa: the recessive partner always passes on a, so the phenotype shows the allele from the tested parent directly
- 2.Recessive offspring occur (about 1 : 1) → the parent also makes a gametes → Aa
- 3.Recessive share: 46 / 94 = 48.94%
Typical mistake: Concluding AA because a result like 52 : 48 is not exactly 1 : 1. Real counts vary; what matters is that recessive offspring appear at all.
Common questions
Why cross with aa in particular?
Because aa only passes on the recessive allele. Then each offspring shows which allele came from the tested parent.