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VA.Fasoula.D.A.Fasoula/Field Crops Research 75 (2002)191-209 195 und ion wei ageneaofi 240 1150 ,1998).Bi 10081 Extensive efforts throughou it the 20th centur ome blocks in the ex improve the vield potential of crons were generally sion of heterosis.confirms the conclusion reached b successful.Research that assessed the yield gains lones (1917)that dominance of linked factors attributed to genetic improvement,revealed an annual accounts successfully for heterosis.He furthermore genetic gain ranging from 0.5 to 1.5%depending on attributed the yield gains in autotetraploid alfalfa to the crop and the conditions ofexp ation(Wych pseudo-overdominance.excluding the presence of nd K and 198 984 (1988 ased the CV of single-plan 1984:Waddington et al..1986:Feil.1992:Tollenaar of two otato cultiv et al..1994:Cox et al..1988).The crops under research were harley (Hordew vulgare l)oats (Avena sativa L.).wheat (Triticum aestivum L.).sor oe e ghum (Sorghum bicolor L.Moench).soybean (Gly CV values of two tomato (Lycopersicon ax (L. Mill.)F2's,whose CV of single- m oved crop com 087 gene Chri 001 The of additive genetic variation clonally ated tato cultivars and the inbre In contrast in crons characterized by nonadditive potato exploits mainly nonadditive allelic variation genetic variation,like the clonally propagated potato whereas that of tomato.additive allelic variation.This (Peloquin,1995).selection for productivity has failed explains why in inbreeders,the genetic advance to esta h ger ic gain.Results reported by Douche through selection is realized even when the genetic bas very pre Phil that a(2000 principal uggested s of the ploid p tato al the difficulty to combine uality and re vide ime to rt re pulsion-in with vield potential and the narrow ge netic has of cultivated potato in North America.We believe deleterious genes This can be accomplished by though.that the principal cause for the reported yield exploiting the advantages of both inbred and hybrid stasis has to be explored in the high load of deleterious vigor.as it is presently done with maize (Duvick. on-pha nkages. 1997 strategy cannot degree of degeneration wher up ne p to the loss of vigor follo heransnong allelic conver loitatio to he Alfalfa is another crop that has failed to Hooker (1968)in maize and Flor (1965)in flax (Lim genetic gain through selection because of the high load usitatissimn i)screened large numbers of progen of deleterious genes.Indeed,the genetic contribution and were able to convert codominant nonfixable to yield in alfalfa over an 80-year period f 0m1898t effects into dominant effects that were fixed upon 1985 was only3%(Hill et al 1988 Holland an inbreeding.These results have far-re hing breeding Bingham,199).Elimination of deleterious genes and implications,because they suggest that the yield o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