disease resistance that is not present in adapted varieties, but it may be used to introduce any
simply inherited trait from a parent that is otherwise undesirable.
After an initial cross between the donor and recurrent parent, repeated backcrosses are
made to the recurrent parent. Each time a backcross is made, the progeny receive half
their alleles from each parent; thus, the proportion of alleles that remain from the donor
parent are reduced by 50% each generation, and afterNbackcrosses, the proportion of
alleles remaining from the donor is formulated as 1/(2Nþ^1 ).^4
After the first hybridization, every backcross will produce a mixture of genotypes at the
locus where we wish to introduce alleles from the donor. Thus, each generation must
produce an adequate number of progeny such that it is possible to identify the heterozygous
genotype carrying the donor allele. While Figure 3.11 shows an example of backcrossing
alleles at a single locus, it is also possible to backcross alleles at two or more loci, but larger
numbers of progeny must be made to identify backcross parents that contain donor alleles at
all loci.
3.4.2.5. Increasing Seed, Testing, and Releasing of Pure-Line Varieties.At
the end of these breeding procedures (typically at the F 5 ,F 6 ,orBC 5 stage), most breeders
will consider that individual plants are adequately homozygous to form a pure line that
(^1) / 2 N+1 D
Figure 3.11.The backcross breeding method is used to transfer alleles at a small number of loci from
a donor parent into the genetic background of a reciprocal parent. Each generation of backcrossing
reduces the proportion of alleles from the donor (D) parent by half^12
, as shown on the right.
(^4) This is true only for loci that are not linked to the locus under selection. Donor parent alleles that are closely linked
to the locus under selection will be highly favored. This phenomenon is called “linkage drag.”
3.4. METHODS OF PLANT BREEDING 67