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Grade 10 · lesson 2 of 4

Infer a Model Parent from an Offspring

Use an offspring genotype to rule out a parent possibility.

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Supplied materials

  • Use the supplied fictional observations or model. Paper and pencil are optional; spoken answers are welcome. No equipment or real experiment is needed.

Learn the idea

Keep the seed’s fictional complete-dominance trait: PP and Pp are purple, pp is white. A purple phenotype alone does not tell whether a parent is PP or Pp. Under this single-gene model, each offspring gets one allele from each parent. An observed pp offspring therefore received a p from both parents.

Suppose a purple parent of unknown genotype is crossed with a known pp parent, and the supplied record includes one white pp offspring. Assume the trait model is exact, parent labels and the observation are correct, no new mutation occurs, and each parent has one of PP, Pp, or pp. The purple parent must be Pp: PP could contribute only P and could not produce pp with this partner. This conclusion depends on the model; it is not a rule for diagnosing real organisms or people.

  1. List PP and Pp as the purple parent’s two possibilities.
  2. For PP × pp, list only Pp offspring.
  3. For Pp × pp, list Pp and pp, each probability 1/2.
  4. Use the supplied pp observation to rule out PP under the stated assumptions.
Worked example

See the method in action

A pp offspring needs p from the unknown purple parent. PP cannot provide p. Because the parent is purple, pp is excluded too; Pp is the remaining genotype in this exact model.

Your turn

Check your reasoning

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If the same cross instead has just one purple offspring and no other observation, what can you conclude?

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See the answer and explanation

Both PP and Pp remain possible.

A Pp offspring could arise from either PP × pp or Pp × pp. One purple observation does not distinguish those parent possibilities.

Take it further

Change the supplied record to include one white pp offspring among three purple offspring. Pp is still the inferred purple-parent genotype; the three purple observations do not cancel the evidence that it contributed p once.

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These LifeStarter passages, scenarios and exercises are authored for this Academy with AI assistance and review. External resources keep their publishers’ terms. A grade label suggests a starting level; it does not establish alignment with your school’s complete requirements.