179recombination is based primarily on single crossovers (Fig. 7.3 ); doubles are so rare
that they may just as well be ignored. Since each recombinant chromosome origi-
nates from a different event, their translocation breakpoints are different. All are
screened for the presence of the desired locus and the position of the translocation
breakpoint. For the second step of the procedure one chromosome from each con-
fi guration is selected, with the desired locus present and the closest available break-
point on the proper side of the locus. The two are combined in a single plant with
Ph1 present, and allowed to recombine. Ph1 permits only homologous recombina-
tion and the only segment of homology in the two primary recombinants is between
the two translocation breakpoints. A crossover in that segment produces two chro-
mosomes: a wheat chromosome with an intercalary alien introgression and a per-
fectly normal alien chromosome.
The size of the introgressed segment depends on the positions of the two break-
points selected for the second round of recombination. Therefore, the precision of
the entire exercise is determined by the number of primary recombinants recovered:
the more are isolated the better the chance of fi nding two fl anking the target locus in
immediate vicinity. The number of primary recombinants in turn depends on the
recombination frequency of the donor and target chromosome arms and the size of
the screened population. If the recombination rate of the two arms is taken as a
Fig. 7.3 Hexaploid wheat homozygous for a recombinant wheat–rye chromosome involving the
distal end of rye chromosome 5RS and wheat chromosome 5DS. Rye segment labeled green ;
wheat chromatin red
7 Introgressions Between Wheat and Rye