Question

Three genes a b c show crossing over $20\%$ between a and b, $28\%$ between b and c and $8\%$ between a and c. Sequence of genes will be.

• A. bac
• B. abc
• C. acb
• D. None of the above

Answer 1

Answer: (A)

bac

Among the genes located on a same chromosome, the recombination% is directly proportional to distance between the genes. If two genes are closely located, then recombination % will be less if they are distantly located, then recombination % will be more.

Steps to solve this:

Step-1)Among the given recombination% values in the question, identify the highest recombination % and the genes .

example: in given question, 28% is the recombination between B & C genes. so, first write B and C with some distance between them.
Step-2 ) observe the second highest recombination % and arrange that accordingly. Example: A & B genes have 20% recombination. Now, two ways are possible:A gene can be towards left to gene B or towards right. To find out correct place, observe the recombination% among A & C, its just 8% . So, genes A & C should be closely placed. Hence, position of Gene A should be in between B & C.
So, the correct answer is 'bac'.