الفهرس 
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Abstract
Objectives:
1-The purpose of this investigation was to study the nature and magnitude of variability generated for different quantitative characters each of the biparental progenies (obtained by attempting intermating of F2 generations) and F3selfed populations.
2-The study was also aimed to estimate the variance components and narrow sense heritability. And also was made to compare the variability of quantitative characters in BIP and F3selfed.
3-The study was extended to predict and isolate the superior segregants or combinations through different selection approaches in cotton populations. In addition to, under standing the shift in association pattern of component characters.
Methods & Results:
In 2010 growing season, the F2 hybrids seeds were planted to obtain F2 plants populations of the three intra-specific crosses as follows:-
Cross I ( Giza 89 × pima ) × 6022. CrossII ( Giza 92 × pima ). CrossIII ( Giza 86 × pima ).
In the same time each male and female F2 plants for each cross were selfed to produce seeds of F3selfed generation.
In 2011 growing season, Biparental populations BIPs derived from inter population matings in F2 generation by using North Carolina Design- I were evaluated with the F3familiesselfed. Two individual field trials were conducted to evaluate the biparental progenies and F3selfs with their original parents for the three crosses, at randomized complete block with three replications.
Different parameters such as analysis of variance , means , range , genotypic and phenotypic coefficients of variability , components of genetic variance , phenotypic variance , heritability in broad and narrow senses , degree of association between different characters and prediction of superior recombination’s were worked out for all the progenies seperately by using Minitap and Spss computer programes.
The salient feature of the present invistigation are summerized as follows:
1- Analysis of variance revealed highly significant mean squares among biparental progenies in the three crosses which may be reflected that the parental involved in these crosses were diverse and this diversity could be transmitted to their progenies.
2- The comparison of mean for different characters among segregating populations indicated that mean values in general were relatively higher in case of biparental populations compared to F3 populations for most characters.
3- The comparison of GCV and PCV among biparental progenies and F3selfed revealed that PCV were generally higher than GCV for all studied characters. This may due to involvement of high genotypic × environment interaction. BIP showed high mean GCV and PCV values for most studied characters compared to F3selfed. This may due to breakage of undesirable linkages and appearance of superior gene combinations. Some characters showed high GCV and PCV such as seed cotton and lint yield and relative growth rate of boll indicating the presence of repulsion phase linkage. However, other characters showed lower GCV and PCV values such as most fiber characters and lint percentage, this may be due to the presence of genes in coupling phase.
4- Higher non-additive genetic variances were noticed for most characters in BIPs populations compared with F3selfed populations, which confermed by high degree of dominance than unity. This may be due breakage of repulsion phase linkage and that may lead to some cryptic genetic chances.
5- The reverse trends were obtained with F3selfed generations, most earliness characters and fiber quality characters were mainly controlled by additive genetic components, but most yield characters were under control non-additive genetic components except for lint percentage and lint index, which affected largely by additive ones.
6- High narrow sense heritability was noticed in F3selfed generation compared with biparental progenies, for most characters. This was due to a great role of additive portion of genetic variance in selfed series. Among biparental populations, the crosses varied in narrow sense heritability with different characters. The characters like, days to first flower, growth period duration, boll maturation period, lint percentage, lint index and most fiber characters showed high narrow sense heritability in both PIB and F3selfed populations. Therefore, selection will be effective for such characters based on phenotypic observation.
7- Acomparison of direction and magnitude of association among characters between biparental and F3selfed populations indicated that several new associations in terms of direction and magnitude were observed. Many additional correlations becomes established in PIBs compared with F3selfed populations. This may occur due to gene reshuffling and breakage of linkage.
8- In general, inter population mating has been very success full in increasing the mean of the population in the desired direction without losing variability. Thus , there is a lot of chance for recombination’s to occur and chance of getting number of superior segregants is also high , the study clearly highlighted the distinct advantage of inter population mating over F3selfed as well as biparental mating in releasing variability in cotton crop.
Conclusion:
The comparison of biparental mating and F3selfed generations shows that the additional variability generated through the inter-mating in early segregating generations has been consequence of release of concealed variability, in the segregating generations, which is probably brought about by rare combinations between the tightly linked genes. And it was also found to be effective in changing not only the magnitude but also direction of correlation coefficients between characters to desired direction.
This approach will help in creating new populations with high frequencies of rare recombinants which can not otherwise be realized in small segregants populations normally being raised through conventional breeding methods especially when desired genes are unfavorably linked. In addition to those, it will also help in maintaining a greater variability for selection to be effective for longer period, and will thus avoid the early fixation of genes in homozygous state. In the same time , estimation of additive and non-additive genetic components were more reliable , thus superior recombinants should be expected in the progenies of biparentalmatings compared with F3 progenies , this due to breaking the repulsion and coupling phase linkages.
Finally, estimates of variance components and nature of gene action can be used for deciding future breeding procedure for improvement populations studied.