The cross - over frequencies between the genes P and Q , P and R as well Q and R are 6% , 15% and 21% , respectively . What is the possible sequence of genes on chromosome ?

Options

A. P, Q, R
B. Q, P, R
C. P, R, Q
D. Either Q, P, R or R, P, O

Answer

To determine the possible sequence of genes on a chromosome based on the crossover frequencies between genes P, Q, and R, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Crossover Frequencies**:
- The crossover frequencies between the genes are given as follows:
- Between P and Q: 6%
- Between P and R: 15%
- Between Q and R: 21%

2. **Convert Crossover Frequencies to Map Units**:
- Crossover frequency can be directly converted to map units (1% crossover frequency = 1 map unit).
- Therefore:
- Distance between P and Q = 6 map units
- Distance between P and R = 15 map units
- Distance between Q and R = 21 map units

3. **Analyze the Distances**:
- The distances indicate that Q and R are the farthest apart (21 map units).
- The distance between P and Q is shorter (6 map units), and the distance between P and R is intermediate (15 map units).

4. **Determine Gene Order**:
- Since Q and R are the farthest apart, we can place them at the ends of the arrangement.
- We need to find a position for P that satisfies the distances:
- If we place Q and R at the ends, the distance between Q and R is 21 map units.
- If P is placed between Q and R, the distance from Q to P should be 6 map units, and from P to R should be 15 map units.
- This arrangement satisfies the condition:
- Q (6 map units) P (15 map units) R = 21 map units.

5. **Final Gene Sequence**:
- The possible gene sequence on the chromosome is Q, P, R (QPR).
- Alternatively, this can also be represented as R, P, Q (RPQ) since the order of genes can be reversed.

6. **Conclusion**:
- The possible sequences of genes on the chromosome are QPR and RPQ.