Agriculture Reference
In-Depth Information
The aim of multiplex breeding is therefore to develop
parental lines which are either triplex or quadruplex
(three or four copies of the desirable allele, respectively)
for the H
1
allele. When crossed to any other parent these
multiplex lines will produce progeny that have at least
a single copy of the H
1
gene and due to dominance, all
will be phenotypically resistant. It will, therefore, not be
necessary to screen for resistance in these progeny. The
ratios of resistant to susceptible amongst the progeny of
genotypes derived by crossing a simplex, duplex, triplex
and quadruplex to a nulliplex are shown in Table 5.5.
Genotype ratios from all possible cross combina-
tions between nulliplex, simplex, duplex, triplex or
quadruplex (Table 5.6)
It was at first thought that developing multiplex par-
ents would be very difficult, instead it proved simply to
be a matter of effort and application. The main dif-
ficulty lies in the fact that progeny tests need to be
carried out to test the genetic make-up of the domi-
nant parental lines (very similar to backcrossing where
the non-recurrent parent has a single recessive gene of
interest). Consider one of the worst situations, where
both starting parents are simplex. Three quarters of the
progeny will be resistant, but only one quarter will be
duplex. So one quarter of the progeny will be nulliplex,
and hence susceptible and so on testing can be imme-
diately discarded. The progeny need, however, to be
testcrossed to a nulliplex to distinguish the duplex from
simplex genotypes in the progeny. Once identified, the
selected duplex lines are inter-crossed or selfed. From
their progeny 1/36 will be quadruplex and 8/36 will be
triplex. A single round test cross will be necessary to
distinguish the triplex and quadruplex lines from those
progeny that are either duplex or simplex. A second
round test cross will be necessary (a second backcross to
a nulliplex lines) in order to distinguish the quadruplex
from triplex lines.
Once quadruplex lines have been identified, these can
be continually inter-crossed, or selfed, without further
need to test. Similarly, a quadruplex or triplex parental
lines can be used in cross combination with any other
parental line and 100% of the resulting progeny will be
Table 5.5
The ratios of resistant to susceptible progeny amongst the genotypes
derived by crossing a simplex, duplex, triplex and quadruplex resistant gene parent
to a nulliplex parent.
Cross type
Phenotype
% Resistant
Resistant : Susceptible
in progeny
Simplex (
Rrrr
)
×
nulliplex (
rrrr
)
1:1
50
Duplex (
RRrr
)
×
nulliplex (
rrrr
)
5:1
83
×
Triplex (
RRRr
)
nulliplex (
rrrr
)
1:0
100
×
Quadruplex (
RRRR
)
nulliplex (
rrrr
)
1:0
100
Table 5.6
Genotype ratios from all possible cross combinations between nulliplex, simplex, duplex, triplex
or quadruplex parents.
Cross
Nulliplex
(N)
Nulliplex
All N
Simplex
(
aaaa
)
(S)
Simplex
1S : 1N
1D : 2S:1N
Duplex
(
Aaaa
)
(D)
Duplex
1D : 4S:1N
1T : 5D : 5S : 1N
1Q : 8T : 18D : 8S : 1N
Triplex
(
AAaa
)
(T)
Triplex
1D:1S
1T:2D:1S
1Q:5T:5D:1S
1Q:2T:1D
Quadruplex
(
AAAa
)
(Q)
Quadruplex
AllD
1T:1D
1Q:4T:1D
1Q:1T
AllQ
(
AAAA
)
