IMAT 2018 Q27 [Crossover Values]

The genes P, Q, R and S occur on the same chromosome. The COV (crossover values) for the pairs of genes are:

P and Q 35%
P and R 5%
R and Q 40%
Q and S 10%
R and S 30%

Which one of the following sequences of letters represents a correct order of genes on the
chromosome?

A. PQRS
B. RSQP
C. PRSQ
D. RPSQ
E. PQSR

The answer to this question is choice D, where the sequence is RPSQ

Do not get intimidated by this question, and approach it like a simple number scale where you have to assign letters where they belong, according to their relative distances, which have been provided to you in the question!

You will need to approach this question systematically to be able to reach the correct answer:

  1. First, you will need to find the 2 ends of the sequence by looking at the letters that have the most relative distance from each other. In this case, R → Q is 40%, where you can safely assume that these are the 2 ends. This is supported by the fact that the other two letters S and P are nearer to R than Q is. At this point, you will need to eliminate the choices that do not have R and Q at the ends, which actually leaves you with choice D as the remaining consideration. But, let us further support our answer using other clues.
  2. Next, find the relative distances of S and P to a reference point, which can be either R and Q since you are certain that they are the 2 ends of the sequence. Thus, look for relative distance to a reference letter that has BOTH S and P, which in this case, the question has given distances of S and P from BOTH R AND Q.
  3. In order to not confuse yourself, it would be best to pick 1 reference point, to figure out the correct sequence. Let’s use R as the reference point. If you look at the numbers, it will be clear that P is nearer to R than S is. This then supports that the correct sequence is RPSQ. To be extra certain, you can use Q as a reference point and check the distances of P and S from Q. If you look at the numbers, S in NEARER to Q than P is.
  4. This then leaves only one answer as correct, which is choice D, or the sequence R P S Q.