Single Choice

Percentage of recombination between A and B is 9%, A and C is 17% and B and C is 26%. The arrangement of genes would be

AA - B - C
BA - C - B
CB - C - A
DB - A - C
Correct Answer

Solution

Since recombination frequency is directly proportional to the distance between genes, the values are used to locate genes on a chromosome. Here, three genes A, B and C can assume any of three linear sequences: A-B-C or A-C-B or B-A-C which depends on of the gene present in the middle. Here, recombination frequency for A and B (X) = 9%, that for A and C (Y) = 17% and for B and C (Z) = 26%. As we can see that Z = X + Y or 26 = 17 + 9 which means that genes B and C are present at extremes and A are in the middle. Thus, the sequence of genes is B-A-C. The correct answer is D. For gene sequence A-B-C, Y=X+Z; but it does not hold true for given values of recombination frequency which make option A wrong. For gene sequence A-C-B or B-C-A, X = Y + Z; but it does not hold true for given values of recombination frequency which make options B and C wrong.

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