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Biology · Grades 9–12

Single-gene inheritance models

Treat dominant as an expression rule, not a claim of superiority.

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Learn the idea

Allele

An allele is a version of a genetic sequence at a locus.

Genotype

Genotype describes allele combinations; phenotype concerns observed traits in a context.

Dominance

In a simple complete-dominance model, one allele’s trait appears in the heterozygote.

Probability

A Punnett square gives modeled probabilities, not guaranteed counts in every small family.

Worked examples

In a stated complete-dominance model, which genotype is heterozygous?

Heterozygous means two different alleles at the locus.

Aa.

For Aa × Aa, what fraction of offspring are expected to be aa in the simple model?

Each parent contributes a with probability one half, giving one quarter.

1/4

Check your understanding

  1. For AA × aa, what percentage of offspring are expected to be Aa?

  2. Does dominant mean more common or biologically better?

    1. Only for every human trait.
    2. Yes, always both.
    3. No.

  3. Why need four offspring not include exactly one aa in an Aa × Aa cross?

Answers and marking notes

Answers and marking notes
  1. 100 %

    Every offspring receives A from one parent and a from the other.

  2. No.

    Dominance describes expression in the specified heterozygote model.

  3. The one-quarter value is a probability, not a fixed small-sample outcome.

    Independent random events can vary around the expected proportion.

Further reading and review references

These are reference links, not copied exercises, institutional endorsement or proof of independent review.

Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.