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  • Topic: Gene expression, Genetics, Hybrid
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  • Published : July 26, 2011
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Hybrid vigor is the increased function of a genetically superior offspring from its parents which normally resulted in increased size, viability, growth rate and productivity in next F1 generation. Charles Darwin for the first time in 1876 described heterosis, which was rediscovered by shull and east in 1908.Shull (1952) defined the heterosis as “the interpretation of increased vigor, size, fruitfulness, speed of development, resistance to disease and to insect pests, or to climatic rigors of any kind, manifested by crossbred organisms as compared with corresponding inbreds, as the specific results of unlikeness in the constitutions of the uniting parental gametes.”.It has also been reported to exhibit the expression of adaptive traits of increased fertility and biotic and abiotic stress resistance respectively. (Dobzhansky, 1950). Genetic models to explain heterosis

Traditional genetic analyses of heterosis are based on the quantitative effects of the genes between two opposite inbred lines. Three major classical genetic models have been suggested to explain the hybrid vigor: the dominance hypothesis explains increased vigor to the action at multiple loci of favorable dominant alleles from both parents, cancelling of deleterious recessives alleles contributed by one parent by dominant alleles contributed by the other parent (Bruce 1910; Jones 1917; Xiao et al. 1995; Cockerham and Zeng 1996, Davenport 1908, Keeble and Pellew (1910) and Jonbes in 1917), overdominance indicated the existence of loci at which the heterozygous state is superior to either homozygotes.Shull 1908; East 1936; Crow 1948; Stuber et al. 1992 (Crow 1948; Hull 1945), and epistasis (Powers 1944; Williams 1959). pseudo-overdominance refers to a particular situation, in which tightly linked genes with favorable dominant alleles in repulsion phase in the parental lines result in an apparent overdominance when combined in the hybrid. Finally, the interaction of favorable alleles from the two parents at different loci, themselves showing additive, dominant or overdominant actions, is taken in account by the epistasis hypothesis (Stuber et al. 1992; Li et al. 2001; Luo et al. 2001).

Johnson and Leefe (1999) report that the Campeloma parthenum has several fold greater survival rate under stressful conditions than one of the parents that crossed to produce it, C. geniculum (Ovate Campeloma). Pseudo-overdominance, refers to condition of tightly linked genes with favourable dominant alleles linked in repulsion. Epistasis involves the interaction of favorable alleles at different loci contributed by the two parents (Yu et al., 1997; Monforte and Tanksley, 2000; Li et al., 2001; Luo et al., 2001). Griffing and Zsiros (1971) explained phenomenon of heterosis by classifying into environment-dependent component parts, such as temperature-dependent heterosis (Langridge, 1962). Riday et al. (2003) suggested that heterosis can sometimes be accounted by the interaction of genes controlling morphologically divergent traits between the parents. Earlier some studies have reported a positive correlation between genetic distance of the parental lines and the superior hybrid performance (Liu et al., 2002; Barbosa et al., 2003). However, in maize (Zea mays), heterosis is known to culminate at an optimum of parental genetic distance before declining again (Moll et al., 1965). Heterosis can be categorized with regard to the deviation of phenotypic traits from the midparent value (MPV) which refers to the average value of a trait in the two parental inbred lines. Midparent heterosis (MPH) describes the deviation of a given trait from the MPV. Best parent heterosis (BPH) refers to phenotypic values above the better performing parent (Hochholdinger and Hoecker 2007).Heterotic effects have been reported for various for various traits, e.g. biomass, size, agronomic yield, pest resistance and tolerance to abiotic stress (Falconer and Mackay 1996).The highest degree of...
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