2-4
Biology 22
Problems in Genetic Mapping
1. Three Hfr strains for E. coli are mated individually with an auxotrophic Fstrain using an interrupted mating procedure. Approximate times of entry
of each gene are listed below. Determine the map of the E. coli
chromosome and show the orientation of the F plasmid in each Hfr strain.
Strain 1
Strain 2
Strain 3
Gene
Entry Time
(min)
Gene
Entry Time
(min)
Gene
Entry Time
(min)
lac+
gal+
his+
argG+
xyl+
Ilv+
thr+
3
12
39
63
74
78
95
argG+
xyl+
Ilv+
thr+
lac+
gal+
his+
19
30
34
51
59
68
95
Ilv+
xyl+
argG+
his+
gal+
lac+
thr+
5
9
20
44
71
80
88
2. An Hfr strain donates the genes xyl+ pro+ lac+ and gal+ to an F- strain.
Recombinants are selected for gal+. Tests are done to determine pre presence
of the other three genes in the gal+ recombinants. What is the gene order?
Gene
Gal+
Lac+
Pro+
Xyl+
Present in
100% of the strains
70% of the strains
30% of the strains
10% of the strains
3. An Hfr strain that is met+ arg+ leu+ strs is conjugated with an F- strain that is
met- arg- leu- strr. Interrupted mating studies show that leu+ enters last.
Recombinants that are leu+ strr are selected and then tested for the presence
of met+ and arg+. The following numbers of bacteria are found for each of the
gentotypes listed below. Determine the gene order and the distances between
the genes in map units.
leu+
leu+
leu+
leu+
metmet+
met+
met-
argargarg+
arg+
50
80
370
0
2-5
4. DNA is isolated from E. coi strain A (his- met- pens) and used to transform
strain B (his+ met+ pens). Transformants are selected on minimal medium +
penicillin to kill his+ met+ cells and survivors are plated on complete medium.
The classes and numbers of cells obtained are listed below. Determine the
recombination frequency between the his and met genes.
Class
his- met+
his+ methis- met-
Number of transformants
35
27
194
5. Three different bacteriophage T2 strains carrying mutations in the r gene (r a, rb
and rc) were each involved in a cross r-xh+ X r+xh-, where x=a, b or c. The
numbers of bacteriophage of each type are listed below. Give any one of four
possible linkage maps for these genes.
r-x h+
r+xh-
r+x h+
r-x h-
r-a h+ X r+a h-
340
420
120
120
r-b h+ X r+b h-
320
560
60
60
r-c h+ X r+c h-
390
590
8
12
6. Transducing phages that infected an A+B+C+ cell are used to infect an A-B-Ccell. Transductants receiving the A+ marker were tested for the presence of B+
and C+. The classes and numbers of transductants observed is shown below.
Determine the gene order and the cotransduction frequencies for A + with B+
and A+ with C+.
Class
A+ B+ C+
A+ B+ CA+ B- C+
A+ B- C-
Number of Transducants
45
80
1
300