Note: The locations of QTLS are not known at the start of this experiment. Strain producing large fruit Strain producing small frult 1A 1A 1B 1B QTL- QTL- - QTL 6B QTL 6A 2A 7A 6A 6B QTL QTL 7B QTL 28 QTL 7A 7B 11A 11B 16B 12B 11A 11B 16A 16A 16B 12A 12A 12B 8A 8A 17A 17A 3B 3B 178 178 ЗА 3A 8B 13A 13A 138 13B 9A 9A 18A 18A 9B 9B 188 18B 4A 4A 14A 14A 4B 14B 14B 19A 19A 198 19B QTL LQTL 208 QTL- QTL- БА SA 10A 10A 15A 15A 20A 20A БВ 5B 10B, 10B 158 158 20B - QTL QTL -a QTL promoting small-size frult QTL QTL QTL- QTL - a QTL promoting large-size fruit Cross the 2 stralins to produce F, offspring. 1A 1B QTL 2A QTL 6A 6B QTL 11A 28 QTL 7A 78 11B 16A 168 12A 12B ЗА 38 8A 8B 17A 17B 13A 13B 18A 188 9A 9B 4A 4B 14A 14B 19A 198 QTL QTL БB 10A - QTL БА 10B 15A 158 20A 20B QTL- Backcross the F,offspring to both parental stralns. Offspring from the backcrosses contaln different combinations of parental chromosomes. For many offspring from the backcrosses, determine frult size and molecular marker composition. 2A, 5A, 114, and 19A are strongly associated with large frult size. This suggests that 4 QTLS lle close to these markers. FIGURE 28.5 The general strategy for QTL mapping via molecular markers. Two different inbred strains have four chromosomes per set. The strain on the left produces large fruit, and the strain on the right produces small fruit. The goal of this mapping strate gy is to locate the unknown genes affecting this trait, which are designated with a QTL label. A black QTL indicates a site carrying one or more allekes that promote large fruit, and a blue QTL carries alkles that promote small fruit. The two strains differ with regard to many molecular markers designated IA and 1B, 2A and 2B, and so forth. The two strains are crossed, and then the Fi offspring are backcrossed to both parental strains. Many offspring from the backcrosses are then ex- amined for their fruit size and to determine which molecular markers are found in their chromosomes. The data are analyzed by computer programs that can statistically associate the phenotype (e.g., fruit size) with particular markers. Markers found throughout the genome of this species provide a way to locate several different genes that may affect the outcome of a single quantitative trait. In this case, the analysis predicts four QTLS promoting heavier fruit weight that are linked to regions of the chromosomes with the following markers: 2A, SA, 11A, and 19A.