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This lesson covers the chi-squared (χ²) test — a statistical test used to determine whether observed results differ significantly from expected results — as required by the Edexcel A-Level Biology specification (9BI0, Topic 8).
When we perform a genetic cross, the observed results rarely match the expected ratio exactly. For example, a 3:1 ratio from 100 offspring might give 78 dominant and 22 recessive instead of the expected 75:25.
The question is: Is this deviation due to chance (random variation), or is it significant (suggesting the expected ratio is wrong)?
The chi-squared test allows us to answer this question objectively.
χ2=∑E(O−E)2
Where:
Before performing a chi-squared test, you must state a null hypothesis:
H₀: There is no significant difference between the observed and expected results.
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