Most tomato fruit quality traits are quantitatively inherited. Many studies have been performed by the groups of S. D. Tanksley (Cornell University, USA) and D. Zamir (The Hebrew University of Jerusalem, Israel) to map QTLs controlling yield and fruit quality related traits (Paterson et al. 1988, 1990, 1991; Azanza et al. 1994; Eshed and Zamir 1995; Goldman et al. 1995; Grandillo and Tanksley 1996b; Tanksley et al. 1996; Fulton et al. 1997, 2000, 2002a; Bernacchi et al. 1998b; Chen et al. 1999; Doganlar et al. 2002c; Frary et al. 2004). These studies were all performed on progeny derived from interspecific crosses between wild tomato species and processing tomato inbreds. Some quality traits of interest for processing tomato are common to fresh market tomato (e.g., sugar content, soluble solids content, pH, acidity, firmness) and QTL locations can be compared across the progenies. In most of the studies QTLs were detected, sometimes with strong effects. A few QTLs explaining a large fraction (20 to 50%) of the phenotypic variation, acting in concert with minor QTLs, are usually detected. Most of the QTLs act in an additive manner, but dominant and overdominant QTLs were detected (Paterson et al. 1988, 1991; de Vicente et al. 1993; Semel et al. 2006). Epistasis (interaction among QTLs) is rarely detected unless a specific experimental design is used (Eshed and Zamir 1996).
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