Agriculture Reference
In-Depth Information
Table 5.2
Expected frequency of genotypes
resulting from a test cross where
TrRr
is
crossed to
ttrr
with independent assortment,
and with coupling and repulsion linkage.
Table 5.3
Observed and expected genotypes
obtained from backcrossing the
F
1
progeny
from a cross between two parents where one
parent has the genotype
AABBCC
and the
other has the genotype
aabbcc
, to the recessive
(
aabbcc
) parent.
Independent
Coupling
Repulsion
assortment
linkage
linkage
Phenotype
Observed
Expected
(
/
)
>(
/
)
<(
/
)
1
4
TR
1
4
TR
1
4
TR
(
/
)
<(
/
)
>(
/
)
1
4
Tr
1
4
Tr
1
4
Tr
A_B_C_
500
149.375
(
/
)
<(
/
)
>(
/
)
1
4
tR
1
4
tR
1
4
tR
aabbcc
510
149.375
(
/
)
>(
/
)
<(
/
)
1
4
tr
1
4
tr
1
4
tr
aaB_C_
50
149.375
A_bbcc
55
149.375
A_bbC_
35
149.375
aaB_cc
38
149.375
from the equation:
A_B_cc
4
149.375
A_bbC_
3
149.375
Recombination %
Total
1195
100
1
This depending on what the initial parents were will be
estimated as:
sum of non-parental classes
total number of individuals
×
=
the same at 149.375 (or the total number of obser-
vations divided by the expected number of phenotype
classes, 1195
/
=
149.375). Obviously, the observed
frequencies are very different from those expected.
The first point to note is that the frequency of the
parental genotypes is considerably higher than expected
and all other
recombinant
types are less than expected.
It is necessary to know the relative order that the
loci appear on the chromosome. The middle locus
can usually be determined from the genotype that is
observed at the lowest frequency. In this example the
lowest observed phenotypes are
A_B_cc
and
aabbC_
,
both of which are effectively parental types at the
A
and
B
locus but involves non-parental combinations with
C
.
As recombination of only the centre gene will involve
a
double cross over
event the frequency of occurrence
will be lowest. Conversely, the pair with most pheno-
typic observed classes for recombinants (not counting
double crossovers) is the outside genes. From this, the
C
-locus would appear to be the middle one.
Consider the possible combinations of alleles and the
frequency that they occur.
For the
A
8
+
TR
tr
=
+
+
+
TtRr
Ttrr
ttRr
ttrr
or
+
Tr
tR
=
+
+
+
ttrr
Linkage can also be detected, but less efficiently, by
selfing the F
1
and observing the segregation ratio of
the F
2
to determine if there is any deviation from
the expectation, based on independent assortment
(e.g. 1
TTRR
:2
TTRr
:1
TTrr
:2
TtRR
:4
TtRr
:2
Ttrr
:1
ttRR
:2
ttRr
:1
ttrr
).
The relative positions of three loci can be
mapped
by considering frequency of progeny from a
three gene
test cross
.
To examine three gene test crosses consider the fol-
lowing cross between two parents where one parent has
the genotype
AABBCC
and the other has the geno-
type
aabbcc
. The F
1
would have the genotype
AaBbCc
.
In order to perform the test cross it is necessary to
cross the F
1
family with a completely recessive genotype
(i.e.
aabbcc
, in this case the recessive parent) and observe
the frequency of the eight possible phenotypes. In our
example the frequencies are as shown in Table 5.3.
If all three loci segregated independently, for exam-
ple they are on different chromosomes, we would expect
the phenotype frequencies of the eight genotypes to be
TtRr
Ttrr
rrRr
−
B
loci we have:
Parental types
A
_
B
_
500
+
4
=
504
aabb
510
+
3
=
513
Recombinants
aaB
_50
+
38
=
88
A
_
bb
55
+
35
=
90
