Question

Construct a chromosome map with the given crossover frequency of each of the genes.

Gene	Frequency of crossover
A-C	30%
B-C	45%
B-D	40%
A-D	25%

• A. ABCD
• B. BADC
• C. CADB
• D. DABC

Answer 1

The correct option is B BADC

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 chromosomal map for the given crossover frequency.
Let us construct the map step by step:

Step 1: Start with the gene that are the farthest apart. B and C are 45 map units apart and must be placed far apart.
B-------------------45%--------------------C

Step 2: Determine the positions of the other genes.
Place the genes logically.
(i) A-C=30
B-------------A-----------30---------------C

(ii)A-D=25
B-------------A--------25--------D---5----C

(iii) B-D=40
B-----15-----A-------25--------D----5----C
<------------40----------------->