Full sibs are generated from biparental crosses
using parents from the base population. The families are evaluated in a
replicated trial to identify and select superior full-sib families, which are
then recombined to initiate the next cycle.
Steps
Cycle 0
_ Season 1. Select random pairs
of plants from the base population and intermate, pollinating one with the
other. Make between 100 and 200 biparental crosses. Save the remnant seed of
each full-sib cross.
_ Season 2. Evaluate full-sib
progenies in multilocation replicated trails. Select promising half-sibs.
_ Season 3. Recombine selected
full-sib. Cycle 1 Same as cycle C0
Genetic issues The genetic gain per cycle is given
by:
where sFS is the phenotypic standard deviation of
the full-sib families.
Application
Full-sib family selection has been used for maize improvement.
Selection response per cycle of about 3.3% has been recorded in maize.
3 Selfed families selection
An S1 is a selfed plant from the base population.
The key features are generations S1 or S2 families, evaluating them in
replicated multi-environment trials, followed by recombination of remnant seed
from selected families.
Steps
_ Season 1. Self pollinate about 300
selected S0 plants. Harvest the selfed seed and keep the remnant seed of each
S1.
_ Season 2. Evaluate S1 progeny rows to
identify superior progenies.
_ Season 3. Random mate selected S1 progenies
to form C1 cycle population. Genetic issues The main reason for this scheme is
to increase the magnitude of additive genetic variance. In theory the genetic
gain is given by:
where sA1 2 is the additive genetic variance among
S1, and sPSA is the phenotypic standard deviation among S1 families. The
additive genetic variation among S2 is
two times that of S1. The S1 and S2, theoretically,
have the highest expected genetic gain per cycle for intrapopulation improvement.
However, various reports
have indicated that, in practice, full-sib and test
cross selection have produced greater genetic gain for both populations per se
and the population crosses.
Application
The S1 appears to be best suited for
self-pollinated Species.It has been used in
maize breeding. One cycle is completed in three
seasons in S1 and four seasons in S2. Genetic gain per cycle 3.3% has been
recorded.
Family selection based on test cross
The key feature of this approach to selection is
that it is designed to improve both the population per se as well as its
combining ability. The choice of the tester is most critical to the success of
the schemes. Using a tester to aid in selection increases the duration of a cycle
by one year. The choice of a tester is critical to the success of a recurrent
selection breeding program. The commonly used testers may be classified into
two:a narrow genetic base tester, and a broad-genetic base tester. Broad-base
testers are used for testing GCA in the population under improvement, whereas
narrow genetic base testers are used to evaluate SCA and possibly GCA. Generally,
plants are selected from the source population and selfed in year 1. Prior to
intermating, the selected plants are crossed as females to a tester in year 2.
Intermating of selected plants occur in year 3.
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