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4 marksPatterns of Inheritance

CAPE Biology Unit 1 · 2005 · Paper 2 · Question 8(b)(ii)

The chi-squared test (x²) estimates the probability that differences between observed (O) and expected (E) results are due to chance. Smooth-seeded (SS dominant) plants were crossed with wrinkled-seeded (ss recessive) plants. F1 progeny were all Ss. The F2 generation produced 90 smooth seeds and 22 wrinkled seeds. The formula for x² is Σ((O-E)²/E).

Once x² is known, how would you use a table of x² values to determine the probability that the difference between observed and expected results is due to chance alone?

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Other parts of this question

  1. 8(a)Using ONE example of epistasis, explain how and why the F2 phenotypes of a dihybrid cross might fail to show the normal 9:3:3:1 ratio for the two characters.[10 marks]
  2. 8(b)(i)Show how you would set out a table to apply the formula: x² = Σ ((O-E)²/E) to find the value of x² where O is the observed and E the expected result.[6 marks]

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