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AP Biology — Unit 5 — Probability & Chi-Square in Genetics — Drill 20

Drill 20 ·

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About This Drill

AP Biology — Unit 5 — Probability & Chi-Square in Genetics — Drill 20 is a practice drill. It contains 5 original questions created by Brian Stewart, a Barron's test prep author with over 20 years of tutoring experience.

Practice applying probability rules and chi-square analysis to genetics problems with this AP Biology drill. You will use the product rule and sum rule to calculate probabilities for genetic crosses, distinguish expected from observed ratios, and interpret chi-square test results to draw conclusions about inheritance patterns.

Passage

Probability is essential for predicting outcomes of genetic crosses. Two fundamental rules apply: Product Rule (AND rule): The probability that two independent events both occur equals the product of their individual probabilities. Example: P(boy AND type A blood) = P(boy) x P(type A) = 0.5 x 0.25 = 0.125 Sum Rule (OR rule): The probability that at least one of two mutually exclusive events occurs equals the sum of their individual probabilities. Chi-Square (χ²) Goodness-of-Fit Test: Used to determine whether observed data differ significantly from expected ratios due to chance alone. Formula: χ² = Σ (O − E)² / E where O = observed count, E = expected count. Critical Values (df = number of classes - 1)
dfp = 0.05 critical value
13.84
25.99
37.82
49.49
If calculated χ² exceeds the critical value at p = 0.05, reject the null hypothesis. If below, fail to reject. Student Experiment: A Pp x Pp cross produced 400 offspring: 315 purple and 85 white plants.

Questions in This Drill

  1. For a monohybrid cross Aa x Aa, what is the probability of obtaining offspring with genotype Aa?
  2. Two genes on different chromosomes: Ee x Ee and Ff x Ff. Using the product rule, what is the probability of offspring with genotype EeFF?
  3. In the student's experiment, a Pp x Pp cross produces 315 purple and 85 white plants out of 400 total. What are the expected numbers under a 3:1 ratio, and what is the calculated chi-square value?
  4. Using the critical value table, what conclusion should the student draw from chi-square = 3.00 for the flower color experiment?
  5. A different student obtained 270 purple and 130 white plants out of 400. The calculated chi-square is 10.67. What conclusion is appropriate?