Higher Tier: construct a genetic cross from evidence
הוצג| English | משמעות |
|---|---|
| Punnett square/ˈpʌnɪt skweə/ | ריבוע פאנאט |
| probability/ˌprɒbəˈbɪlɪti/ | סיכוי |
What would explain this observation?
- Higher Tier: Two dominant-looking parents have a recessive offspring in a stated simple model. Their appearance alone was not enough to identify their genotypes.
Build the model
- Start with the allele key.
- In a completely dominant single-gene model, a recessive offspring has genotype aa and must receive an a allele from each parent.
- If both parents show the dominant phenotype, each also has A, so both are Aa.
- List each parent’s possible gametes, A and a, before filling the cross.
- Punnett square 庞尼特方格: A grid combining possible parental gametes.
- probability 概率: The chance of an outcome under the stated model.
Under the stated model, what does a recessive offspring of two dominant parents show?
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
A recessive offspring (aa) needs an a allele from each parent, so both dominant-showing parents must carry a hidden recessive allele - they are Aa. Two AA parents could not produce aa.
Match each technical term to its precise meaning.
רמת קושי 2/5 · 140 מטבעות + בונוס קטן בהשלמה הראשונה היום
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Each box combines one allele from each parent: AA, Aa, Aa and aa.
- State genotype and phenotype probabilities separately.
- The inference depends on the stated model: one gene, complete dominance and no new mutation.
- Do not apply it unchanged to characteristics controlled by many genes or to a scenario with incomplete dominance.
In the lab: Write the key, infer parental genotypes using the evidence, label gametes along the grid edges and combine them. Show each step so a mistaken parental assumption is visible. Compare the predicted distribution with supplied observed counts; small samples often deviate from expected proportions without disproving the model.
Which two habits make the investigation or model in this case more defensible?
רמת קושי 2/5 · 140 מטבעות + בונוס קטן בהשלמה הראשונה היום
Stating the conditions and assumptions lets others repeat the work exactly, and recording the supporting evidence lets them check the conclusion. Hiding disagreeing results removes the evidence that could challenge a wrong conclusion, and one measurement can never establish a wider claim.
Work from known quantities
- Known: an Aa parent crossed with aa produces Aa, aa, Aa, aa.
- Recessive probability is 1/2.
- In 60 independent model offspring the expected count is 30.
- For two independent offspring, probability both are recessive is 1/2×1/2 = 1/4.
- Multiplication applies because the prompt states independent outcomes.
Your turn: An Aa×aa model gives recessive probability 1/2. Find the expected number in 90 offspring.
An Aa×aa model gives recessive probability 1/2. Find the expected number in 90 offspring.
רמת קושי 2/5 · 140 מטבעות + בונוס קטן בהשלמה הראשונה היום
The result is 45 offspring. Known: Aa x aa gives recessive probability 1/2; 90 offspring. Expected recessive = 90 x 1/2 = 45 offspring. Expectation is not a guarantee for any actual litter.
Check the conclusion and its limits
- Writing only a 3:1 ratio earns no explanation of where it came from.
- Aa×aa does not give 3:1.
- A recessive outcome is possible again even when the previous offspring was recessive; the model has no memory of earlier births.
Not true: Aa×aa gives the same phenotype ratio as Aa×Aa. Aa x Aa gives a 3:1 phenotype ratio, while Aa x aa gives 1:1. The crosses differ in the second parent's genotype, so the ratios cannot be the same.
- Start with the allele key.
- Each box combines one allele from each parent: AA, Aa, Aa and aa.
- Watch for: writing only a 3:1 ratio earns no explanation of where it came from.
Aa×aa gives the same phenotype ratio as Aa×Aa.
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
Aa x Aa gives a 3:1 phenotype ratio, while Aa x aa gives 1:1. The crosses differ in the second parent's genotype, so the ratios cannot be the same.
A grid combining possible parental gametes: write the technical term.
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
Punnett square means A grid combining possible parental gametes.