Probability — Science definition
The proportion of an outcome expected over many offspring — not a guarantee about any small number of them.
Where it is taught
- Inheritance, Probability & Punnett Squares — Genes, Cell Division & Heredity, Life Science & Scientific Inquiry, Grade 7
In use
A Punnett square sets out the gametes from each parent along two edges and fills in every combination, giving the probability of each outcome.
Inheritance, Probability & Punnett Squares
Questions using probability
Problem 1
Answer and explanation
The expected genotype probabilities are 50% Aa and 50% aa; the recessive phenotype probability is 50%. Four independent offspring need not contain exactly two of each. These predictions depend on the fictional single-region model and supplied dominance rule; they are not general predictions for complex human traits.
Aa contributes A or a; aa contributes a.
Combinations are Aa and aa with equal probabilities.
Apply the stipulated phenotype rule and separate probability from count.
Response criteria
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.
Problem 2
Answer and explanation
No. Small independent samples can differ from expected ratios. Examine more evidence and the assumptions before choosing between sampling variation and a different model.
Response criteria
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.