Understanding gene expression, segregation and their inheritance from parent to offspring had been one of the major goals in developmental genetics because, determination of the superior parents and hybrid combinations before their use in breeding program is useful for a wide range of quantitative characteristics and suggested the contributions of different types of genetic effects to heterosis. For this reason, the attempts to determine the relative contribution of different kinds of gene effects (additive, dominance and epistasis) to genetic variance components of the economic traits has become a primary focus for poultry research. Consequently, most of genetic studies demonstrated that epistatic effects have been increasingly recognized as important genetic factors in the inheritance of quantitative traits because epistatic variance can shift to additive variance under inbreeding, also it contributes to heterosis in crossbreeding. Moreover, heterotic effects includes variability produced by interaction between loci (epistasis) and interaction within heterozygous loci (dominance including over-dominance). Epistatic deviation is termed by the difference between the observed phenotype and predication based on genotype and/or it describes the deviations that occur when the combined additive effect of two or more genes does not explain an observed phenotype. Moreover, there was a correlation between the presence or absence of heterosis and the presence or absence of epistasis, while heterosis can rise in the absence of epistasis. Physiological epistasis contributes to additive, dominance and interaction genetic values and variances (Crow and Kimura 1970). Generally, the magnitude of heterosis was positively correlated with the contribution of additive x additive gene interaction, which would be useful in improving the selection responses of the economic traits through its conversion to additive genetic variance (Goodnight, 1988). Also, it can be used in developing the new commercial poultry lines (Subbaraman and Rangasamy, 1989).
TYPES OF EPISTATIC EFFECTS
1. Additive x Additive: presents additive x additive non allelic interaction. Kinghorn (1983) found that only additive x additive type of epistasis would provide an adequate general description of epistatic variance. Furthermore, the analysis for testing the presence of epistatic effects revealed that presence of highly significant additive x additive type of epistasis in the inheritance of some economic (Abou El-Ghar and Abdou, 2004; Abou El-Ghar et al., 2007 & 2010 & 2011)
2. Additive x Dominance: denote to additive x dominance non allelic interaction. Hagger et al. (1986) ; Fairfull et al. (1987) found significant additive x dominance effects for different traits in laying hens. Abou El-Ghar and Abdou (2004) found that additive x dominance epistatic effects were highly significant in the inheritance of body weight at sexual maturity, egg number at the first 90 d., of production and number of egg till 52 wks., of age.
3. Dominance x Dominance: denotes to dominance x dominance type of non allelic interaction (epistatic effects). Such type of gene inter action reflects a minor effects on quantitative traits.
METHODS OF ESTIMATING THE EPISTATIC EFFECTS
Many experiments have been carried out on the inheritance of quantitative differences, some of kind, which can lead to partition of the variance into its components (additive, dominance and environmental effects), this partition of variance shows how much of it is (1) heritable and fixable differences between homozygote´s (2) heritable but unfixable differences between heterozygote´s and the mean of pairs of homozygote´s (3) non-heritable and serving the genetical situation. The original generation mean analysis proposed by Mather (1949) and Hayman (1958) measured all genetic variations, but before any genetic analysis can be carried out, scaling test has to be done to test the presence or absence of epistasis (Mather, 1949; Hayman and Mather, 1955). Thus, generation mean analysis were proposed as a technique for estimating the component of generation mean and heterosis as well as determining the directions corresponding to dominance, epistatic and additive gene effects (Jinks and Jones, 1958). Under the null of hypothesis that there were no epistatic effects, Hayman (1958) proposed a model for describing the components of generation means and variances. When epistasis was absent a model comprising three parameters(general mean, additive and dominance effects) my be used to describe the generation means and variances (Jinks and Jones, 1958). Also a method of Ketata et al., (1976) for partitioning of variance into its components based on the deviation between the phenotypic value of the 3-way cross and the mean of its parents (single crosses). The variation of line sums and differences were used to estimate additive and dominance mean squares. In this respect, epistatic effects were partitioned into: additive x additive type of epistasis together with both additive x dominance and dominance x dominance types of epistasis.
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