Agriculture Reference
In-Depth Information
to the ratio of 9
AB
:3
Ab
:3
aB
:1
ab
. Similarly, we find
the genetic ratio of 1
AB
:1
Ab
:1
aB
:1
ab
, on test cross-
ing the F
1
to a complete recessive (aabb). Any deviation
from these expected ratios is an indication of linkage
between the two gene loci.
Consider a second simple example of recombina-
tion frequencies derived from a test cross, and how
the test cross can be used to estimate recombination
ratios and hence ascertain linkage between characters.
Consider the dwarfing gene in spring barley and a
third single gene, which confers resistance to barley
powdery mildew. Mildew resistant genotypes are des-
ignated as
RR
and susceptible genotypes as
rr
. A cross is
made between two barley genotypes where one parent is
homozygous tall and mildew resistant (i.e.
TTRR
); and
the other is short and mildew susceptible (i.e.
ttrr
). We
would expect the F
1
to be tall and resistant (i.e.
TtRr
).
When the heterozygous F
1
was crossed to a completely
recessive genotype (i.e.
ttrr
) we would expect to have
a 1:1:1:1 ratio of phenotypes. When this cross was
carried out and the progeny from the test cross was
examined the following frequencies of phenotypes and
genotypes were the ones actually observed.
in only 20% of meiotic events, or a frequency of 0.1
for each type. Knowing the frequency of four gamete
types we can now proceed and predict the frequency of
genotypes and phenotypes we would expect in the F
2
generation using a punnet square.
Gametes from male parent
Gametes from
female parent
−
−
−
−
TR
0
.
1
Tr
0
.
4
tR
0
.
4
tr
0
.
1
TR
−
0
.
1
TTRR
TTRr
TtRR
TtRr
0.01
0.04
0.04
0.01
Tr
−
0
.
4
TTRr
TTrr
TtRr
Ttrr
0.04
0.16
0.16
0.04
tR
−
0
.
4
TtRR
TtRr
ttRR
ttRr
0.04
0.16
0.16
0.04
tr
−
0
.
1
TtRr
Ttrr
ttRr
ttrr
0.01
0.04
0.04
0.01
Collecting like genotypes results in:
Tall and resistant
=
T_R_
=
0.66
=
=
Short and resistant
ttR_
0.09
=
=
Tall and susceptible
T_rr
0.09
=
=
0.16
From a breeding standpoint compare these pheno-
typic ratios to those that would have been expected
with no linkage (i.e. 0.56
T_R_
: 0.19
ttR_
: 0.19
T_rr
: 0.06
ttrr
). If, as might be expected, the aim was
to identify individual plants which were short and resis-
tant, then the occurrence of these types would drop
from 19% to 9%, by more than half. It should also be
noted that 10% recombination is not particularly high.
Overall, therefore, genes on the same chromosome
(particularly same chromosome arm) are linked and
do not segregate independently. Two heterozygous loci
may be linked in coupling or repulsion. Coupling is
present when desirable alleles at two loci are present
together on the same chromosome and the unfavourable
alleles are on another (e.g.
TR/tr
). Repulsion is present
when a desirable allele at one locus is on the same
chromosome as an unfavourable allele at another locus
(e.g.
Tr/tR
).
In the test cross
TtRr
Short and susceptible
ttrr
Tall and Resistant (
TtRr
)
=
79
Short and Resistant (
ttRr
)
=
18
Tall and Susceptible (
Ttrr
)
=
22
=
Short and Susceptible (
ttrr
)
81
=
Total
200
Linkage is obviously indicated by these results as the
observed frequencies are markedly different from those
expected at 50 : 50 : 50 : 50, on the basis of indepen-
dent assortment. The two phenotypes,
tall and resistant
and
short and susceptible
, occur at a considerably higher
frequency than we expected. You will readily note that
these are the same phenotypes as the two parents. The
other two phenotypes (the non-parental types) were
observed with much lower frequency. Added together,
the two recombination types (
short and resistant
, and
tall
and susceptible
) only account for 40 (20%) out of the
200 F
2
progeny examined, while we expected their con-
tribution to collectively make up 50% of the progeny.
Therefore the F
1
plants are producing
TR
gametes or
tr
gametes in 80% of meiotic events, or a frequency of 0.4
for each type. Similarly
Tr
or
tR
gametes are produced
×
ttrr
the situations shown in
Table 5.2 can arise.
The percentage recombination, expressed as the
number of map units between loci, can be calculated
