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Grade 10 · lesson 1 of 4

Use a simple allele probability model

Distinguish model probabilities from a guaranteed number of offspring.

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Supplied materials

  • Fictional plant trait model: P and p are two alleles. Under this deliberately simplified complete-dominance model, PP and Pp have purple flowers; pp has white flowers. Each parent is Pp and contributes P or p with equal probability.
  • Assume independent allele contributions in this model. Do not use it to predict a person’s traits.

Learn the idea

An allele is a version of a gene. In a simple complete-dominance model, one P allele is enough for the modelled purple trait. Dominant does not mean better, stronger or necessarily more common.

List one allele from each parent to make four equally likely combinations. Probabilities describe chances under the assumptions; four offspring do not have to include exactly one of each combination. Many real traits involve more complex inheritance.

  1. Draw a two-by-two table with P and p from each parent.
  2. Fill the combinations PP, Pp, pP and pp.
  3. Count the combinations with no P allele.
  4. Use the changed parent pair in the question.
Worked example

See the method in action

Pp × Pp gives PP, Pp, pP and pp. Three of four combinations contain P, so purple has probability 3/4 and white has probability 1/4 under this model.

Your turn

Check your reasoning

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If the parents are instead Pp and pp, what is the probability of white flowers under the same model?

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See the answer and explanation

1/2

The Pp parent contributes P or p equally; the pp parent always contributes p. Half the combinations are Pp and half are pp, so white has probability 1/2. This is a chance per offspring, not an exact family count.

Take it further

For PP × pp, every combination is Pp and the model gives purple probability 1. Explain why this model still should not be generalized to every real trait.

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Longer courses for learners ages 14+

These related courses have their own preparation and projects. They are not early-grade assignments.

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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.