Linkage Example
Example:
In Drosophila, black body (b) and vestigial wings (vg)
are on chromosome II.
Black Body
Vestigial Wings
r = 18.5 Map Units (centiMorgans)
Experiment
Make the following cross
b+
b+
vg+
vg+
b
b
X
The F1 is
b+
b
vg+
vg
vg
vg
Test Cross
F1
X
b
b
vg
vg
In order to do the Punnett Square, we need to look at
the gametes that F1 individuals produce.
Gametes
b+
vg+
b
vg
b+
vg
b
vg+
Probability
½ (1-r)
½ (1-r)
½r
½r
The Punnett Square is:
Females
Males b+
vg+ b
b
vg b+b vg+vg bb
vg b+
vg b
vg+
vg vg b+b vg vg bb vg+vg
The Punnett Square is:
Females
Males b+
vg+ b
vg b+
vg b
vg+
b
vg b+b vg+vg bb vg vg b+b vg vg bb vg+vg
Prob.
½ (1-r)
½ (1-r)
½r
½r
Example ½ (1-.185) ½ (1-.185) ½ (.185) ½ (.185)
0.4075
0.4075
0.0925
0.0925
Important
In the Test Cross, the frequencies of the phenotypes of
the offspring are the same as the frequencies of the
gametes produced by the non-test parent (in this case,
the F1).
Linkage with 3 loci
Example:
In Tesuque tomato plants, three loci control the shape
and color of the tomatoes. The dominant trait is listed
on top (and the recessive is listed on the bottom).
Red Fruit
Yellow Fruit
Large Leaves
Small Leaves
10
Coefficient of Coincidence = 0.80
Lobed Leaves
No Lobes
15
The Experiment:
A plant from a pure breeding strain of yellow small
lobed plants is crossed with a plant from a pure
breeding strain of red large and non-lobed plants.
P0: Yellow Small Lobed X Red Large Non-lobed
The resulting F1 is:
F1:
Yellow
Red
The phenotype is:
Small
Large
Lobed
Non-lobed
F1:
Yellow
Red
Small
Large
Lobed
Non-lobed
The gametes are:
Gametes Produced
Yellow Small Lobed
Red Large N-L
Crossover Event
Yellow
Small
Lobed
Red
Large
N-L
Red Small Lobed
Yellow Large N-L
Yellow
Small
X
Red
Large
Yellow Small N-L
Red Large Lobed
Yellow
Red
Yellow Large Lobed
Red Small N-L
Yellow
Small
Lobed
X
X
Red
Large
N-L
Lobed
N-L
Small
Lobed
X
Large
N-L
Probabilities of Gametes Produced:
Gametes Produced
Probability
Yellow Small Lobed 0.381 = ½{ 1 - 0.10 - 0.15 +
Red Large N-L 0.381 = 0.8(0.10)(0.15)}
Red Small Lobed 0.044 = ½ { 0.10 Yellow Large N-L 0.044 = 0.8 (0.10) (0.15) }
Yellow Small N-L 0.069 = ½ { 0.15 Red Large Lobed 0.069 = 0.8 (0.10) (0.15) }
Yellow Large Lobed 0.006 = ½ { 0.8 (0.10) (0.15) }
Red Small N-L 0.006 =
Remember,
For a Test Cross, the expected frequencies for the
phenotypes will be the expected frequencies for the
gametes produced by the F1 individual.
Expected Frequencies for the Dihybrid Cross
Note: Symbols on the outside of the Punnett Square refer to gametes,
those within the Punnett Square refer to phenotypes.
0.381
YSL
0.381
RLgN
0.044
RSL
0.044
YLgN
0.069
YSN
0.069
RLgL
0.006
YLgL
0.006
RSN
0.381
YSL
0.14516
YSL
0.14516
RLgL
0.01676
RSL
0.01676
YLgL
0.02629
YSL
0.02629
RLgL
0.00229
YLgL
0.00229
RSL
0.381
RLgN
0.14516
RLgL
0.14516
RLgN
0.01676
RLgL
0.01676
RLgN
0.02629
RLgN
0.02629
RLgL
0.00229
RLgL
0.00229
RLgN
0.044
RSL
0.01676
RSL
0.01676
RLgL
0.00194
RSL
0.00194
RLgL
0.00304
RSL
0.00304
RLgL
0.00026
RLgL
0.00026
RSL
0.044
YLgN
0.01676
YLgL
0.01676
RLgN
0.00194
RLgL
0.00194
YLgN
0.00304
YLgN
0.00304
YLgN
0.00026
YLgL
0.00026
RLgN
0.069
YSN
0.02629
YSL
0.02629
RLgN
0.00304
RSL
0.00304
YLgN
0.00476
YSN
0.00476
YSN
0.00041
YLgL
0.00041
RSN
0.069
RLgL
0.02629
RLgL
0.02629
RLgL
0.00304
RLgL
0.00304
RLgL
0.00476
RLgL
0.00476
RLgL
0.00041
RLgL
0.00041
RLgL
0.006
YLgL
0.00229
YLgL
0.00229
RLgL
0.00026
RLgL
0.00026
YLgL
0.00041
YLgL
0.00041
RLgL
0.00004
YLgL
0.00004
RLgL
0.006
RSN
0.00229
RSL
0.00229
RLgN
0.00026
RSL
0.00026
RLgN
0.00041
RSN
0.00041
RLgL
0.00004
RLgL
0.00004
RSN