There are three genes a, b, and c. Percentage of crossing over between a and b
is 20%, b and c is 28% and a and c is 8%. What is the order of arrangement of
these three genes on the chromosome?
a, b, c
a, c, b
b, a, c
c, b, a
Correct Gene Order
The correct arrangement of genes a, b, and c on the chromosome is a, c, b. This order fits the recombination data where a and c show the lowest crossing over at 8%, indicating closest proximity, while b lies farthest from both.
Recombination frequency measures physical distance between genes, with lower percentages signaling closer linkage. Here, a-c (8%) represents the shortest distance. The sum of a-c (8%) and c-b (20%) approximates a-b (20%), confirming c between a and b. Similarly, b-c (28%) matches the total span from b to c via a (though double crossovers slightly reduce observed rates).
Option Analysis
a, b, c: Invalid. Expected a-c distance would be a-b (20%) + b-c (28%) = 48%, but observed a-c is only 8%, a major mismatch.
a, c, b: Correct. Distances align: a-c = 8%, c-b ≈ 20% (from a-b data), total a-b ≈ 28% (close to observed 20-28%, accounting for interference).
b, a, c: Invalid. Predicted b-c = b-a (20%) + a-c (8%) = 28%, matching observed, but a-b should be shortest if central—contradicts data where b-c is highest.
c, b, a: Invalid. Expected c-a = c-b (28%) + b-a (20%) = 48%, far exceeding observed 8%.
| Option | Predicted a-c | Observed a-c | Valid? |
|---|---|---|---|
| a, b, c | 48% | 8% | No |
| a, c, b | 8% | 8% | Yes |
| b, a, c | 28% | 8% | No |
| c, b, a | 48% | 8% | No |
Gene Mapping Principles
In three-point crosses, the gene with recombination frequencies summing closest to distant pairs occupies the middle position. Double crossovers (rare) can underestimate distances, but the smallest frequency always identifies adjacent genes. For CSIR NET preparation, practice verifying arrangements by adding segment distances and checking against all pairs.
