Three Point Mapping Experiment: Backward Analysis

1
Three Point Mapping Experiment Backward Analysis

• In a three-point cross, the following are
  essential
• Knowledge of whether genes/loci are autosomal or
  X-linked, dominance relationship.
• Heterozygous individual (heterozygous at all
  loci) to produce gametes.
• Accurate determination of genotypes of gametes
  that are produced by the heterozygous individual.
• Sufficient number of progeny from the linkage
  testcross so that all crossover classes are
  represented.
• Determine gene order, map distances between
  genes, coefficient of coincidence and interference

2
Three Point Mapping Backward Analysis
wing morphology cu curled wings curled
upward cu () straight wings wing
angle tx taxi wings 75 from axis tx ()
wings not held out bristles gro groucho
clumps above eyes gro () no extra
bristles Three autosomal loci are linked, but
suppose gene order is unknown. Conduct linkage
crosses as shown on the right.
cu tx gro

(P)

cu tx gro
Linkage Testcross
cu tx gro
cu tx gro
(F1)

cu tx gro
Generate progeny. Suppose n 1000
3
Three Point Mapping Backward Analysis Progeny
Phenotypes/Gametes from Heterozygote
Phenotype
Counts
Class
Phenotype
Counts
Class
cu tx gro
269
NCO
cu
201
?
593
397

324
NCO
tx gro
196
?
cu tx
1
?
cu gro
3
?
3
7
gro
2
?
tx
4
?
GENE ORDER The locus in the middle is the one
that differs from the parentals.
A B C
A b C
A b C
DCO class is expected to have the fewest number
of progeny.
a b c
a B c
a B c
4
Three Point Mapping Backward Analysis
Phenotype
Counts
Class
Phenotype
Counts
Class
cu gro tx
269
NCO
cu
201
SCO I

324
593
gro tx
196
397
cu tx
1
DCO
cu gro
3
SCO II
gro
2
3
tx
4
7
Region cu - gro gro - tx cu - tx
Recombination Frequency (397 3)/1000 0.40 or
40 (7 3)/1000 0.01 or 1
Map Distance 40 m.u. 1 m.u. 41 m.u.
Coefficient of Coincidence (C) observed DCO
3 0.75 expected
DCO (0.4)(0.01)(1000) I gt 0 means fewer obs DCO
than expected. I lt 0 means more obs DCO than
expected.
Interference (I) 1 - C I 1 - 0.75 0.25