Worked example
In an invented plant, A is dominant over a. Which allele pair is heterozygous, meaning two different versions?
Answer & why
Answer: Aa
Aa contains two different allele versions.
Grade 9 · Science
Use a simple inheritance model
Use stated allele rules and equally likely model outcomes without generalizing a single-gene model to every trait.
6 authored questions + worked example
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All questions
All 6 authored questions for this skill.
Question 1
For Aa × Aa, the four equally likely model pairs are AA, Aa, aA and aa. What fraction has two lowercase a alleles?
- 1
- 3/4
- 1/2
- 1/4
A hint
List the possible allele pairs and use only the dominance rule given for this invented plant trait.
Answer & why
Answer: 1/4
Only aa is one of the four listed outcomes.
Question 2
For Aa × aa, the equally likely pairs are Aa, Aa, aa and aa. If A is dominant, what fraction shows the A-associated trait in this model?
- 1/2
- 1/4
- 1
- 3/4
A hint
List the possible allele pairs and use only the dominance rule given for this invented plant trait.
Answer & why
Answer: 1/2
Two of the four listed pairs contain A.
Question 3
A model gives a 1/4 probability of aa for each offspring. Must exactly one of every four actual offspring be aa?
- No; probabilities do not fix every small group's outcome.
- No; aa is impossible.
- Yes; every four offspring must match exactly.
- Yes; probability prevents variation.
A hint
List the possible allele pairs and use only the dominance rule given for this invented plant trait.
Answer & why
Answer: No; probabilities do not fix every small group's outcome.
Independent chance outcomes can vary in small groups.
Question 4
A single-gene model is useful for a stated plant trait. Which conclusion would go beyond its evidence?
- Every human characteristic follows this same simple rule.
- Its predictions depend on its assumptions.
- The model uses the two stated allele versions.
- Its listed outcomes can be counted.
A hint
List the possible allele pairs and use only the dominance rule given for this invented plant trait.
Answer & why
Answer: Every human characteristic follows this same simple rule.
A simplified example does not establish a universal model of complex traits.
Question 5
In an invented plant, A is dominant over a. Which allele pair is heterozygous, meaning two different versions?
- AA
- aa
- A only
- Aa
A hint
List the possible allele pairs and use only the dominance rule given for this invented plant trait.
Answer & why
Answer: Aa
Aa contains two different allele versions.
Question 6
In this model, a parent with Aa can pass A or a, each with probability 1/2. Which contribution can it make to one offspring?
- Only an allele B not present in the model.
- Both alleles together as its one contribution.
- Only AA.
- Either A or a.
A hint
List the possible allele pairs and use only the dominance rule given for this invented plant trait.
Answer & why
Answer: Either A or a.
The rule supplies one of its two alleles to an offspring.