Advantages
_ It is an easy and rapid
way to attain homozygosity.
_ Small spaces are required
in early generations to grow theselections.
_ Natural selection has no effect.
_ The duration of the breeding program can be reduced
by several years by using single seed descent.
_ Every plant originates from a different F2 plant,
resulting in greater genetic diversity in each generation.
_ It is suited to environments that do not
represent those in which the ultimate cultivar will be commercially produced.
Disadvantages
_ Natural selection has no effect.
_ Plants are selected based on individual phenotype
not progeny performance.
_ Inability of seed to germinate or plant to set
seed may prohibit every F2 plant from being represented in the subsequent
population.
_ The number of plants in the F2 is equal to the number
of plants in the F4. Selecting a single seed per plant runs the risks of losing
desirable genes. The assumption is that the single seed represents the genetic
base of each F2. This may not be true.
Modifications
The procedure described so far is the classic
single seed descent breeding method. There are two main modifications of this
basic procedure. The multiple seed procedure entails selecting 2–4 seeds per plant, bulking
and splitting the bulk into two, one for planting the next generation, and the
other half held as reserve. Because some soybean breeders simply harvest one
multiseeded pod per plant, the procedure is also referred to by some as
bulk-pod method. Another modification is the single hill method in which
progeny from individual plants are maintained as separate lines during the
early generations by planting a few seeds in a hill. Seeds are harvested from
the hill and planted in another hill the next generation. A plant is harvested
from each line when homozygosity is attained.
Backcross breeding
This application in plants was first proposed by
H.V. Harlan and M.N. Pope in 1922. In principle, backcross breeding does not
improve the genotype of the product, except for the substituted gene.
The rationale of backcross breeding is to replace a
specific undesirable gene with a desirable alternative, while preserving all
other qualities of an adapted cultivar.
Instead of inbreeding the F1 as normally done, it
is repeatedly crossed with the desirable parent to retrieve the desirable genotype.
The adapted and highly desirable parent is called the recurrent parent in the
crossing program, while the source of the desirable gene missing in the adapted
parent is called the donor parent. Even though the chief role of the donor
parent is to supply the missing gene, it should not be significantly deficient in
other desirable traits. An inferior recurrent
parent will still be
inferior after the gene transfer.
The backcross method of breeding is best suited to improving
established cultivars that are later found to be deficient in one or two
specific traits. It is most effective and easy to conduct when the missing
trait is qualitatively inherited,
dominant, and produces a phenotype that is readily observed in a hybrid plant.
Quantitative traits are more difficult to breed by this method. The procedure
for transferring a recessive trait is similar to that for dominant traits, but
entails an additional step. Backcrossing is used to transfer entire sets of
chromosomes in the foreign cytoplasm to create a cytoplasmic male sterile genotype
that is used to facilitate hybrid production in species including corn, onion,
and wheat. This is accomplished by crossing the donor as male until all donor
chromosomes are recovered in the cytoplasm of the recurrent parent.
Backcrossing is also used for the introgression of genes
via wide crosses. However, such programs are often lengthy because wild plant
species possess significant amounts of undesirable traits. Backcross breeding
can also be used to develop isogenic lines BREEDING SELF-POLLINATED SPECIES 327
for traits in
which phenotypes contrast. The method is effective
for breeding when the expression of a trait depends mainly on one pair of
genes, the heterozygote is readily identified, and the species is self-fertilizing.
Backcrossing is applicable in the development of multilines.
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