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Genetics Question of the Day

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Tuesday, August 11, 2026

The distance between genes pr and vg in Drosophila is 13.0 cM, and the distance between vg and b is 6.3 cM. The gene order is pr-vg-b. In a test cross of a pr vg b / + + + female, 10 double-crossover flies were observed among 2000 total progeny. What is the interference?

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The distance between genes pr and vg in Drosophila is 13.0 cM, and the distance between vg and b is 6.3 cM. The gene order is pr-vg-b. In a test cross of a pr vg b / + + + female, 10 double-crossover flies were observed among 2000 total progeny. What is the interference?

  1. 1.64
  2. 0.61
  3. 0.39 (correct answer)
  4. -0.64

Explanation: When you encounter genetics problems involving map distances and crossovers, you're dealing with chromosome mapping and interference. Interference measures how one crossover event affects the probability of another crossover occurring nearby. To solve this, you need to compare expected versus observed double crossovers. The expected number of double crossovers equals the product of individual crossover frequencies times total progeny: 0.13×0.063×2000=16.380.13 \times 0.063 \times 2000 = 16.38 expected double crossovers. The coefficient of coincidence (COC) is: observedexpected=1016.38=0.61\frac{\text{observed}}{\text{expected}} = \frac{10}{16.38} = 0.61 Interference is calculated as: Interference=1−COC=1−0.61=0.39\text{Interference} = 1 - \text{COC} = 1 - 0.61 = 0.39 This means 39% interference occurred, so answer C is correct. Let's examine why the other answers are wrong: Answer A (1.64) likely results from incorrectly calculating interference as expected/observed instead of 1 - (observed/expected). Answer B (0.61) is actually the coefficient of coincidence, not interference - this is a common trap where students confuse these related but distinct concepts. Answer D (-0.64) might come from a sign error or misunderstanding the interference formula. Remember that interference always ranges from 0 to 1, representing the degree to which one crossover reduces the probability of another nearby crossover. When you see mapping problems, always distinguish between coefficient of coincidence (observed/expected) and interference (1 - COC).