Question

There are three genes a, b, 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 sequence of genes on chromosome?

• A. b, a, c
• B. a, b, c
• C. a, c, b
• D. None of the above

Answer 1

The correct option is A b, a, c

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 the gene present in the middle. Here, recombination frequency for a and b (X) = 20%, that for a and c (Y) = 8% and for b and c (Z) =28%. As we can see that Z = X + Y or 28 = 20+8 which means that genes b and c are present at extremes and a is in the middle. Thus, the sequence of genes is b-a-c.