Question

There are three genes: a, b, and c. The 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 chromosomes?

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

Answer 1

The correct option is A b, a, c

A linkage/crossover/chromosome map is a graphic representation of relative positions/order and relative distances of genes in a chromosome in the form of a line like a linear road map depicting different places and their relative distances without giving exact mileage. It is based on Morgan’s hypothesis (1911) that the frequency of crossover/recombination between two linked genes is directly proportional to the physical distance between them. One map unit or centrimorgan is equivalent to 1% recombination between two genes.
The percentage of crossing over between a and b is 20%, so they are 20 map units apart, whereas b and c are 28 map units apart. So, the correct sequence of genes on chromosomes will be as follows:
b ----20---------------a------8---- c