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Significant deviation of 'b'
from zero and the non-significant departure of regression coefficient
from unity in respect of days to heading, peduncle length, number of
grains per spike, grain weight per spike, 1000 grain weight and grain
yield indicated that the aforesaid diallel assumptions were valid for
these traits (Table 2).
However, rest of the characters showed partial failure of the
assumptions but estimates of the population parameters for that
traits were still possible (Hayman 1954) though certainly the
estimates for such a trait are less reliable than they would have
been if all assumptions had been fulfilled. With the fulfillment of
most of the assumptions of the diallel analysis fully or partially in
the present study, the conclusions drawn are expected to be valid and
should form a guideline for improvement in the genetic material
studied.
The estimates of components of genetic variance (Table
2) exhibited that additive component (D) was highly significant
for all the characters except days to maturity. The two measures of
dominance H1 (dominance effect) and
H2(proportion of dominance due to positive and negative
effect of genes) were also highly significant for all the traits
studied. Thus, it is suggested that additive and non-additive gene
effects were equally important for all the characters except days to
maturity, for which only non-additive component was important. These
findings confirm the results of Jatasra and Paroda (1980), Sharma and
Singh (1982), Nanda et al. (1983), Sharma et al. (1986), Dasgupta and
Mondal (1988), Solanki et al. (1993) and Singh et al. (1993) obtained
through combining ability analysis for the same set of
characters.
The estimates of 'F' value
which indicated the relative frequency of dominant and recessive
alleles in parents were found to be positive and highly significant
for days to heading, number of grains per spike, grain weight per
spike, 1000 grain weight, harvest index and grain yield indicating an
excess of dominant alleles, while 'F' was significant and negative
for plant height, which indicated the directional dominance of the
decreasing genes. Positive but non- significant 'F' in days to
maturity, flag lead area and tiller number gave some indications of
the excess of dominant alleles in the parental lines. The
environmental component (E) was significant for days to maturity,
flag leaf area, grain weight per spike and grain yield. The
proportion (H1/D)1/2 representing the degree of
dominance was more than unity for all the characters indicating the
existence of overdominance.
The present study revealed that both additive and non-additive
components of genetic variances were involved in governing the
inheritance of yield and yield components, although preponderance of
non-additive genetic variance was noted. In such a situation, the
most suitable breeding procedure would be one which mops up the
additive genetic variance and at the same time maintains
heterozygosity. Therefore, it is desirable to practice bi-parental
mating and/or recurrent selection, intermating the most desirable
segregants, alternately with selection. This would lead to an
elevation of the genetic plateau, by accumulating favorable additive
genes and simultaneously exploiting the dominance variance. Although
it is difficult to produce enough seed in wheat by conventional
methods, the 'obligate' cross-fertilization system using male
sterility, as proposed by Athwal and Borlaug (1967) can bring about
large-scale intermating among selected genotypes, as envisaged under
the recurrent improvement programme.
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