Three genes R, S and T are located on the same chromosome. If the recombinant percentage between R and S is 20%, R and T is 35% and S and T is 15% respectively, can you predict the correct order of these genes on the chromosome ? Which of the following shows the correct sequence of the genes on the chromosome ?

The correct option/sequence is R - S - T.

To determine the correct order of genes on a chromosome (gene mapping), we use the recombination frequencies between the genes. In gene mapping, the recombination percentage (recombination frequency) is directly proportional to the physical distance between the genes on the chromosome. By convention, 1% recombination frequency is equal to 1 map unit (mu) or centimorgan (cM).

Based on the provided data, we have the following distances:
- Distance between R and S = 20% recombination = 20 map units
- Distance between R and T = 35% recombination = 35 map units
- Distance between S and T = 15% recombination = 15 map units

To find the correct sequence, we start with the two genes that are furthest apart, which represent the outer boundaries. Here, the distance between R and T is the largest (35 map units). This indicates that R and T are the terminal genes on this segment of the chromosome:
R ----------------------------------------- T (Distance = 35)

Next, we place the third gene, S, relative to R and T using their respective distances. The distance between R and S is 20 map units, and the distance between S and T is 15 map units. Since:
$20+15=35$
the gene S must lie between R and T.

Thus, starting from R, we travel 20 units to reach S, and then another 15 units to reach T. This perfectly matches the overall R to T distance of 35 units:
R --------(20 units)-------- S ----(15 units)---- T

Therefore, the linear arrangement of the genes on the chromosome is R - S - T.