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Science · Grades 8–11

Genetics: models and probability

These questions assume one gene with alleles A and a, complete dominance, and equally likely allele transmission.

Make a worksheet

Understand the idea

State the model

These questions assume one gene with alleles A and a, complete dominance, and equally likely allele transmission.

Do not overinterpret

Dominant does not mean common, stronger, better or certain in an individual child.

Examples and reasoning

In the stated model, which genotype is heterozygous?

Aa

Aa

Which genotype is homozygous recessive?

aa

aa

Check your understanding

  1. An Aa parent can contribute which allele types in this model?

    1. no allele
    2. Aa only
    3. A or a
    4. AA only

  2. For Aa × aa, what fraction of expected offspring genotypes are aa? Give a fraction.

  3. For Aa × Aa, what fraction of expected offspring genotypes are AA?

  4. For Aa × Aa, which genotype class has the largest expected fraction?

    1. all are equally likely
    2. aa
    3. AA
    4. Aa

Answers and marking notes

Answers and marking notes
  1. A or a

    A or a

  2. 1/2

    1/2

  3. 1/4

    1/4

  4. Aa

    Aa

Further reading and review references

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

Learning preview · teacher review required. Grade bands are approximate, not official curriculum alignment.