2-6
Biology 22
Problems in Genetic Mapping
1. Three different Hfr strains of E. coli transfer their genes in the order and
with entry times given below. Using these results, construct a linkage map
of the circular E. coli chromosome. Express the gene distances in transfer
time (minutes) between adjacent pairs of genes.
Strain A
Strain B
Strain C
Gene
Entry Time
(min)
Gene
Entry Time
(min)
Gene
Entry Time
(min)
Mal+
Strs
Ser+
Ade+
His+
3
14
21
44
58
Ade+
His+
Gal+
Pro+
Met+
9
23
48
56
68
Pro+
Met+
Xyl+
Mal+
Strs
7
19
24
41
52
2. A prototrophic Hfr strain of E. coli with the genotype arg+ bio- leu+ is conjugated
with an F- strain with the genotype arg- bio+ leu-. The arg gene is known to
enter last. The following numbers of recombinants were isolated:
arg+ bio- leu+
420
arg+ bio+ leu65
+
+
+
arg
bio leu
15
arg+ bio- leu0
Draw a genetic map that shows the order in which these genes are arranged
and the distances between them.
3. E. coli cells have been infected with two strains of T4 bacteriophage. One
strain has the characteristics of minute (m), rapid-lysis (r), and turbid (tu); the
other is wild type (+) for all three markers. The genotypes for the bacteriophage
produced by this mixed infection are shown in the table below. Draw a genetic
map that shows the order in which these three genes are arranged and the
distances between them.
Genotype
Number of
Bacteriophage
m r tu
2796
+ + +
2882
m + +
1384
+ r tu
1438
m + tu
628
+ r +
594
m r +
146
+ + tu
132