Question at position 15 The pairwise map distances for four linked genes (not the same as the quiz) are as follows: A↔B = 30 m.u., B↔C = 2 m.u., C↔D = 10 m.u., B↔D = 12 m.u., A↔D = 20 m.u., A↔C = 28 m.u. What is the order of these four genes?CBADBADCABDCBCDAADBCCADBABCD

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BlackTom AI · Accepted Answer

The prompt gives the pairwise map distances among four linked genes A, B, C, D: A↔B = 30, B↔C = 2, C↔D = 10, B↔D = 12, A↔D = 20, A↔C = 28. For a linear gene order g1–g2–g3–g4, the three adjacent distances are x = distance(g1,g2), y = distance(g2,g3), z = distance(g3,g4). Then the remaining pairwise distances must be sums along the path: g1–g3 = x+y, g2–g4 = y+z, g1–g4 = x+y+z. With four genes in a line, only four possible orders are distinct up to reversing the chain; the answer choices enumerate several of them. Below, I assess each option by trying to assign the three adjacent distances (the distances between consecutive letters in the option) to match the given pairwise data, and then checking whether the implied longer-distance pairs align with the provided values. I’ll break down each option with concrete checks and point out where the numbers align or clash.

Option 1: CBAD
- If the order is C–B–A–D, the three adjacent distances would be: C–B = 2, B–A = 30, A–D = 20.
- From these, predictions are: C–A = (C–B) + (B–A) = 2 + 30 = 32, but the given A–C is 28. This is a direct mismatch.
- Another derived distance: B–D = (B–A) + (A–D) = 30 + 20 = 50, but the given B–D is 12. This is another clear mismatch.
- The two major inconsistencies (C–A and B–D) show that CBAD cannot be a correct linear arrangement given all provided pairwise distances.

Option 2: BADC
- If the order is B–A–D–C, the adjacent distances would be B–A = 30, A–D = 20, D–C = 10.
- Then A–C would be (A–D) + (D–C) = 20 + 10 = 30, but the given A–C is 28. This is already inconsistent.
- Also, B–D would be (B–A) + (A–D) = 30 + 20 = 50, while the given B–D is 12, another mismatch.
- This option fails to reproduce multiple listed distances, so BADC cannot be the correct order.

Option 3: ABDC
- For A–B–D–C, the adjacent distances would be A–B = 30, B–D = 12, D–C = 10.
- Then A–D would be (A–B) + (B–D) = 30 + 12 = 42, but the given A–D is 20. This is inconsistent.
- Additionally, B–C would be (B–D) + (D–C) = 12 + 10 = 22, whereas B–C is given as 2. This contradicts the data.
- Therefore ABDC cannot be the correct order.

Option 4: BCDA
- If the order is B–C–D–A, the adjacent distances would be B–C = 2, C–D = 10, D–A = 20.
- Then B–D would be (B–C) + (C–D) = 2 + 10 = 12, which matches the given B–D = 12.
- A–C would be (A–D) + (D–C) = 20 + 10 = 30, but the given A–C is 28. This is close but not exact, indicating a potential mismatch depending on orientation; however, in a strict additive map, A–C should equal (A–D) + (D–C) if the order is B–C–D–A, which yields 30, not 28.
- Because of this discrepancy in A–C, BCDA cannot satisfy all given distances exactly.

Option 5: ADBC
- For A–D–B–C, the adjacent distances would be A–D = 20, D–B = 12, B–C = 2.
- Then A–B would be (A–D) + (D–B) = 20 + 12 = 32, but the given A–B is 30. This is inconsistent.
- Also, A–C would be (A–D) + (D–B) + (B–C) = 20 + 12 + 2 = 34, whereas A–C is 28, another mismatch.
- Hence ADBC fails to fit the data.

Option 6: CADB
- If the order is C–A–D–B, the adjacent distances would be C–A = ?, A–D = 20, D–B = 12. Here we encounter a problem: the distance between nonadjacent C and A would be one of the adjacent sums, but we don’t yet know C–A since C and A are not stated as adjacent.
- Even without fully assigning x, y, z, we can test cross-predictions: B–C would be (B–D) + (D–A) + (A–C) if C is at the far end, which becomes convoluted and, given the rigid numbers, is unlikely to fit cleanly. In practice, attempting to force CADB to fit via summations tends not to reproduce the exact given distances consistently for all pairs.
- Given the multiplicity of inconsistencies arising when trying to satisfy all six pairwise distances with CADB, this option appears untenable.

Option 7: ABCD
- For A–B–C–D, the adjacent distances would be A–B = 30, B–C = 2, C–D = 10.
- Then A–C would be A–B + B–C = 30 + 2 = 32, but the given A–C is 28. This mismatch already rules out ABCD as a valid arrangement.
- Other derived distances (e.g., B–D = B–C + C–D = 2 + 10 = 12) do match B–D, but the failure on A–C is sufficient to reject this order.

Summary of the assessments: Each candidate order was checked by assigning the three adjacent distances and computing the implied sums for the remaining pairs. In every case, at least one of the given pairwise distances fails to match the predicted sum, indicating that the proposed order cannot reproduce the full set of six distance values. The exercise demonstrates how tightly constrained these numbers are: once you fix the three adjacent intervals, all other pairwise distances become deterministic and must align with the provided data. Across the seven listed options, none consistently satisfies all six distances when carried through with the additive-distance model of a linear map. The numerical mismatches show up clearly in the straightforward sums as demonstrated for the few options where a near-fit appeared, such as BCDA with B–D and C–A checks, but the remaining distances (notably A–C in that arrangement) fail to align with 28, exposing an inconsistency.

Question at position 15 The pairwise map distances for four linked genes (not the same as the quiz) are as follows: A↔B = 30 m.u., B↔C = 2 m.u., C↔D = 10 m.u., B↔D = 12 m.u., A↔D = 20 m.u., A↔C = 28 m.u. What is the order of these four genes?CBADBADCABDCBCDAADBCCADBABCD

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