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Cotton Genomics and Genetics
14
The combining ability describes the breeding value of
parental lines to produce hybrids, general and specific
combining ability as defined by (Sprague and Tatum,
1942) who stated that
gca
effects were due to an
addi
tive type of gene action, but
sca
effects were due
to genes which exhibit non additive (dominant and
epistatic) type of gene action. Combining ability
analysis helps in the evaluation of inbreds in terms of
their genetic value and in the selection of suitable
parents for hybridization. The superior specific cross
combinations were also identified by this technique.
General combining ability is the average performance
of a genotype in cross combinations involving a set of
other genotypes. It is specific for the set of lines and
testing environment, whereas specific combining
ability is the average performance of a specific cross
combination expressed as deviation from the
population mean (Sprague and Tatum, 1942). The
gca
effect reflects the breeding value of the parental
genotypes and assists in identifying genotypes to be
used for developing superior populations. Specific
combining ability effects represent the non-reliable
component of the genotypic value arising due to
contribution from dominance deviation and interaction
deviation. Hence,
sca
effect is the main cause for
superiority of a cross. It is inferred that superiority of
a cross can not be fixed through selection.
Basal et al. (2009) studied eight barbadense cotton
cultivars and fifteen F
1
hybrids obtained by crossing
five lines and three testers in the line x tester mating
system. The predominance of non-additive gene
action was estimated for all the traits except for seeds
per boll, which were controlled by additive type of
gene action due to high GCA variance.
El- Mansy et al. (2010) hybridized nine diverse
G.barbadense
cotton genotypes i.e.; Pima S6 as
American Egyptian cotton, Karshenky 2 as Russian
genotypes, Suven as Indian genotype and six Egyptian
genotypes i.e. Monoufi, dandara, Giza 86, Giza87,
Giza 89 x Pima S6 and Giza 92, following half diallel
crossing system in order to investigate the genetic
mechanism controlling variation. Analysis of
combining ability revealed significant
gca
and
sca
for
most studied characters indicating importance role of
additive and non-additive effects in the inheritance
these characters.
Mohammad Reza Zangi and Nadali Bbaein Jelodar
(2010) determined combining ability and heterosis in
crosses of
G.hirsutum
and
G.barbadense
for agro
morphological traits and yield. High variation was
observed for characteristics among parents and the F
1
combinations. Barbadense 5539 and Termeze14
(
G.barbadense
) had negative
gca
for plant height,
bolls per plant and sympodia branches per plant.
Barbadense genotypes also showed negative
gca
for
monopodia per plant and boll weight. The GCA: SCA
ratios for the studied traits were higher than one
indicating the presence of additive genetic effects for
most of the characteristics studied.
The term heterotic group refers to “a group of related
or unrelated genotypes from the same or different
populations, which display similar combining ability
and heterotic response when crossed with genotypes
from other genetically distinct germplasm groups”
(Melchinger and Gumber, 1998).
In the recent years the concept of developing heterotic
populations is put to test in self pollinated crops like
cotton, segregating populations based on diverse pairs
of genotypes can be the ideal base material required
for implementing procedures like reciprocal selection
for improving combining ability (Patil and Paltil, 2003
and Patil et al., 2011). In hybrid research study on
cotton, a large number of crosses involving varietal
lines are used for assessing combining ability status.
On constantly observing the most potential crosses
attempts are made to infer about the causes of high
heterosis.
Utilization of heterosis depends on genetic diversity
existing between the parents, magnitude of dominance
at the yield influencing loci and the genetic distance
between the chosen parental genotypes. It is possible
to maximize heterosis by enhancing genetic distance
between two chosen parental populations. Many
population improvement schemes are followed in
cross pollinated crops to increase genetic diversity, to
create heterotic groups and exploit them. These
schemes can be extended to self pollinated crops by