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Gene Mapping
I.
GOALS
A. Determine the sequence (order) of genes on the chromosome
B. Determine the spacing between genes on the chromosome
II.METHOD
A. Recombinational method
1. Double infect bacteria with phage containing portions of the chromosome
a) The phage should contain chromosome portions containing different alleles
for the genes
(1) Example, one phage might be A+ and B-, while the second phage will be Aand B+
2. Analyze recombination frequency
a) Recombinational frequency is proportional to distance between gene
B. Linkage and multifactor crosses
1. Definitions
a) Linkage
(1) Two genes very close to each other so recombination between them would be
very rare
b) Multifactor
(1) Looking at three or more genes at once
2. Method
a) Create a A+ . . . B- . . . C- and a A- . . . B+ . . . C+ phage
b) Double infect
(1) Use phage that will not be replicated on its own, but needs the other phage
c) Select for all A+C+ recombinants
d) Ask if they are also B+
(1) If many are B+, then B is closer to C than A
(2) If many are B-, then B is closer to A than C
e) If B and C are linked, there are two possibilities
(1) A . . . . . BC or A . . . . . CB
3. Determining order
a) Create an A+ . . . B- . . . C+ and a A- . . . B+ . . . C- phage
b) Look for A+B+C+ recombinant
(1) If the order is ABC, it would take to recombinational events which would be
quite rare
(a) About 0.1%
(2) If the order is ACB, recombination would be more frequent
(a) About 1%
4. Complementation
a) Phenotypes may be a result of several gene products
b)
c)
d)
e)
(1) Complementation may be used to determine the number of genes are
regulatory elements
(a) Mutation in any of these could prevent the phenotype from being
expressed
Double infect bacteria with various mutants lacking a phenotype
(1) If the phenotype is displayed, the mutations were in different genes
(a) A-B+C+ / A+B-C+
(2) If the phenotype is not displayed, both phage carry the same mutated gene
(a) A-B+C+ / A-B+C+
Can determine which gene is mutated in a new mutant (X)
(1) A-B+C+ / X-; A+B-C+ / X-; A+B+C- / X(2) If phenotype is not displayed, the mutation is in the same gene
(3) If the phenotype is displayed, mutation must be in a different gene
(4) If phenotype is displayed in all cases, you've discovered a new gene
Reasons why complimenting might not work
(1) They are mutated in the same gene
(2) Mutation is in the regulatory site for the other gene
(3) Mutation yields an inhibitory product
Complementation groupings
(1) Group mutants by the gene that is mutated
(2) Complementation groups will not compliment each other