Q.96 If the recombination frequency between X – Y loci is 12, X – Z loci is 4, and Y – Z loci is 8, then the
order of the loci on the chromosome is
(A) X-Y-Z (B) Y-X-Z (C) X-Z-Y (D) Z-Y-X
Determining Gene Order from Recombination Frequencies: X-Y 12%, X-Z 4%, Y-Z 8%
Recombination frequency measures genetic distance between loci on a chromosome, where lower values indicate closer proximity. For loci X, Y, and Z with frequencies X-Y (12%), X-Z (4%), and Y-Z (8%), the correct order is X-Z-Y as it aligns additively: 4% + 8% = 12%.
Core Concept
Recombination frequency approximates map distance in centimorgans (cM), assuming 1% recombination = 1 cM. For three loci, the correct linear order shows the largest distance as the sum of smaller segments between flanking genes.
This avoids underestimation from double crossovers in direct parent-offspring comparisons.
Correct Answer: (C) X-Z-Y
X and Z are closest (4%), Z and Y next (8%), so X-Z-Y yields X-Y distance of 4% + 8% = 12%, matching observed data exactly.
No discrepancies arise, confirming Z between X and Y.
Option Analysis
| Option | Predicted Distances | Matches Data? | Reason |
|---|---|---|---|
| (A) X-Y-Z | X-Y: 12%, Y-Z: 8%, X-Z: 20% | No | X-Z should be 12% + 8% = 20%, but observed is 4% |
| (B) Y-X-Z | Y-X: 12%, X-Z: 4%, Y-Z: 16% | No | Y-Z should be 12% + 4% = 16%, but observed is 8% |
| (C) X-Z-Y | X-Z: 4%, Z-Y: 8%, X-Y: 12% | Yes | 4% + 8% = 12%; all match |
| (D) Z-Y-X | Z-Y: 8%, Y-X: 12%, Z-X: 20% | No | Z-X should be 8% + 12% = 20%, but observed is 4% |
Why Additive Distances Matter
The farthest pair (X-Y at 12%) must encompass intermediates; closest pair (X-Z at 4%) anchors one end. Test orders by summing segments against observed values—only X-Z-Y fits without excess.
This principle applies universally in three-point crosses for linkage mapping.
