Linkage, Crossing Over, and Mapping in Eukaryotes

1
Chapter 6 Linkage, Crossing Over, and Mapping in
Eukaryotes
Problems 2-10, 12-15, 20, 24, 26
2
In chickens
W white F frizzled feathers w colored f
normal feathers
3
x
wwFF
WWff
P
gametes
wF
Wf
WwFf
X
wwff
F1
18 63 63 13
recombinant
WwFf
parental
Wwff
gametes
parental
wwFf
recombinant
wwff
Mendels 2nd rule Chromosomes assort
independently
4
Linked (yes cross over)
1/4
1/4
unlinked
1/4
1/4
parental
recombinant
Linked (no cross over)
1/4
1/4
A and B are in the same linkage group!
linkage groups haploid of chromosomes
1/4
1/4
all parental gametes
Fig. 6.1
5
A
no crossing over
W
f
f
W
nonsister chromatids
W
f
W
f
tetrad
w
F
F
w
w
F
sister chromatids
w
F
4 parental gametes
B
nonsister chromatid crossing over
W
f
parental gametes
f
W
chiasma
W
F
f
recombinant gametes
W
w
f
w
recombinant gametes
F
w
F
w
F
parental gametes
C
sister chromatid crossing over
f
W
f
W
f
W
parental gametes
W
f
F
w
F
w
w
F
w
F
6
Chromosome Mapping
Originally developed by Thomas Hunt Morgan and
Alfred H. Sturtevant.
Map distance 1 map unit 1 centimorgan (cM)
1 chance of crossing over
Back to our chickens
parental recombinants F2 offspring
126
31
157
31/157 20 chance of cross over
So, the two genes are 20 cM apart (20 map units).
7
Actual
W
f
63 40.1 18 11.5 13 8.3 63 40.1
40 10 10 40
f
W
20 chance of a cross over.
W
F
W
f
w
F
f
w
w
F
w
F
20cM
W
f
20cM
w
F
8
Only crossover events between genes can be
observed.
Crossover event outside the 2 genes. NO
recombinants seen!
Crossover event between the 2 genes. YES
recombinants seen!
Fig. 6.5
9
Multiple Crossovers
A
A
B
A
b
single cross over
a
b
B
a
B
A
B
A
B
double cross over
a
b
a
b
C
A
B
A
B
C
c
double cross over
a
b
a
b
c
C
10
Three-Point Crosses
3 Criteria

• The individuals being tested need to be
  heterozygous at the loci being studied.
• Genotypes of the gametes are easily determined by
  looking at the phenotype of the offspring.
• Large number of offspring need to be looked at.
  Crossing over occurs at a low frequency between
  genes that are close together.

11
Three-Point Cross
parentals
recombinants
10,000 offspring total.
Fig. 6.8
12
Determining Gene Order
Method 1
Arrangement III is correct!
Fig. 6.9
13
Determining Gene Order
Method 2 Look at two genes at a time.
Gene order
14
Calculation of Map Distance
Final Map
15
Another three-point cross!
In corn v virescent seedling gl glossy
seedling va variable sterility
v gl va gl va va v
va v gl va gl v gl v

normal gl, va va v, va v, gl, va gl v, gl v
235 62 40 4 270 7 48 60 726
Parental
Single-crossover
Single-crossover
Double-crossover
Parental
Double-crossover
Single-crossover
Single-crossover
16
Calculation of map distance
Distance from v to gl?
Single-crossovers
133 / 726 18.3
gl va v
62 60 4 7 133
Double-crossovers
v va gl
Distance from gl to va?
Single-crossovers
99 / 726 13.6
v gl va
48 40 4 7 99
Double-crossovers
v va gl
17
Map Distance (continued)
Distance from v to va?
Single-crossovers
232 / 726 31.9
gl va v
62 60 48 40 4(2) 7(2) 232
v gl va
Double-crossovers
v va gl
Double-crossovers are counted twice!
va
gl
v
Final Map
13.6 cM
18.3 cM
31.9 cM
18
If the single-crossovers occur independent of
each other then
freq. of double c/o (freq. single c/o)(freq.
single c/o)
(0.183)(0.136) 2.5 (expected)
But the actual of double-crossovers 11 /
726 1.5
observed double c/o expected double c/o
Coefficient of Coincidence (C)
C 1.5 / 2.5
0.6
Interference 1-C
I 1 0.6 0.4