If the arrangement of genes A, B, C and D on any of the chromosomes is ‘ABDC’, then which among the following is the incorrect statement?

1. The approximate distance between genes A&B can be 20m.u, B&C 30m.u, C&D 10m.u and A&C 50m.u.
2. The approximate distance between genes A&B can be 15m.u, B&C 35m.u, C&D 5m.u and A&C 50m.u.
3. The approximate distance between genes A&B can be 31m.u, B&C 10m.u, C&D 5m.u and A&C 4m.u.
4. The approximate distance between genes A&B can be 22m.u, B&C 28m.u, C&D 8m.u and A&C 50m.u.

The correct option is C The approximate distance between genes A&B can be 31m.u, B&C 10m.u, C&D 5m.u and A&C 4m.u.
Alleles of a particular gene are present at discrete loci on the chromosome. The distance between two gene loci on a chromosome is expressed in terms of a unit known as CentiMorgan (cM) or map unit (m.u).
Greater the distance between the genes, lesser are those genes linked, greater is the frequency of recombination.
Now, in this case, since the sequence of the genes is given, hence, the relative distance between a pair of genes can be obtained easily.
The sequence of the gene on the chromosome is ABDC.
Since the gene A and C are located at the farthest, the distance between them is the maximum.
Let's consider that maximum value as 50m.u.
The gene A and B are located closer to each other than A and C, hence, the distance between A and B is less than the distance between A and C.
Let's consider the distance between A and B genes is x m.u and the distance between A and C genes is 50 m.u.
The diagram below shows the possible distance between the genes.
Based on this, the only incorrect option is-
The distance between genes A&B can be 31m.u, B&C 10m.u, C&D 5m.u and A&C 4m.u.
In this case, the distance between A and B and that between B and C is not matching to that of the distance between A and C. (31m.u + 10m.u = 41m.u is not matching with 9 m.u).