The map distance between genes A and B is 3 units, between B and C is 10 units and between C and A is 7 units. Which of the following shows the correct order of the genes in a linkage map on the above data ?

Options

A. A,B,C
B. A,C,B
C. B,C,A
D. C,A,B

Answer

To solve the problem of determining the correct order of genes A, B, and C based on the given map distances, we can follow these steps:

### Step 1: Identify the Distances
We have the following distances:
- Distance between A and B = 3 map units
- Distance between B and C = 10 map units
- Distance between C and A = 7 map units

### Step 2: Determine the Farthest Genes
From the distances provided, we can see that the greatest distance is between B and C (10 map units). This suggests that B and C are the farthest apart in the linkage map.

### Step 3: Position the Farthest Genes
We will place gene B on one end and gene C on the other end of our linkage map:
- Position: B — 10 map units — C

### Step 4: Determine the Position of Gene A
Next, we need to find the position of gene A. We know that the distance between A and B is 3 map units. Therefore, we can place gene A 3 map units from gene B:
- Position: A — 3 map units — B — 10 map units — C

### Step 5: Verify the Distance Between A and C
Now, we need to check the distance between A and C. The distance between C and A is given as 7 map units. If we add the distances:
- From A to B = 3 map units
- From B to C = 10 map units

The total distance from A to C through B would be:
3 (A to B) + 10 (B to C) = 13 map units, which does not match the given distance of 7 map units.

### Step 6: Adjust the Position of Gene A
Since the distance from A to C is only 7 map units, we need to adjust the position of gene A. Thus, we can place A between B and C:
- Position: B — 3 map units — A — 4 map units — C

This configuration satisfies all the distances:
- Distance between A and B = 3 map units
- Distance between A and C = 7 map units (3 + 4)
- Distance between B and C = 10 map units (3 + 4 + 3)

### Conclusion
The correct order of the genes in the linkage map is:
- B — A — C

### Final Answer
The correct order of the genes is B, A, C.

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