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11/01/11
Group Activity
You need to map a newly identified mutation m by the end of your rotation in the Ashrafi lab. You decided to use a
mapping strain homozygous for all three known genetic markers a, b and c.
a-Choromome I
b-Chromosome II
c-Chromosome III
You know that:
1) m, a, b and c are recessive loci
2) a, b and c have BamHI restriction site.
Allele
A
a
B
b
C
c
Before BamHI digestion
300bp
300bp
100bp
100bp
200bp
200bp
After BamHI digestion
300bp
230bp and 70bp
100bp
50bp and 50bp
200bp
150bp and 50bp
 After mating m/m strain with a test strain, you isolated F1 hermaphrodites.
You got an A in your undergraduate genetics course and; as a result, you know that although all of F1 progeny is WT it
is heterozygous for all four loci.
 You took F1 and put them on separate plates for self-fertilization.
1



You isolated m/m F2 progeny to figure out which of the three markers is linked to your mutation.
You performed PCR to amplify all three marker loci.
You cut the PCR products with BamHI and ran the fragments on a gel.
You got the following result:
2
On which chromosome the mutant allele is found?
Chromosome I.
Since it rarely segregates with a.
Even though it segregates with c only 67% of the time, which is smaller than
75%, it is likely due to a small sample size and not because of the linkage.
If mutation is not linked to a marker—it will segregate ~75% of the time
mutant
1
2
3
4
5
6
7
8
9
10
AA
AA
Aa
AA
AA
AA
Aa
AA
AA
AA
GENOTYPE
BB
bb
Bb
bb
bb
Bb
Bb
Bb
BB
Bb
cc
CC
cc
Cc
CC
Cc
Cc
Cc
CC
Cc
3
11
12
AA
AA
BB
bb
CC
Cc
What is the map position of the mutation?
Map units (cM) = 100*(# of recombinant chromosomes/total # chromosomes)
2/24*100 = 8 cM
4