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Chapter 3. A more accurate adjustment could be made if these F2 plants were progeny tested to calculate the degree of penetrance involved when these parents are used. This method would be preferred, because it has been reported that the level of penetrance of a trait can change when the genetic background is changed (25, 35). When a chi-square test for goodness-of-fit was conducted on the adjusted frequency, Florunner x PI 445924 was the only population that was significantly different from the expected frequency. This difference could have been caused by sampling error, a different level of penetrance than reported in Chapter 3, or by a different mode of inheritance than the one proposed. Further studies are needed to determine which of these explanations is best.
Based on the model described by Nigam et al. (39), the expected frequencies in the F2 and chi-square test would be the same as those presented in Table 6-4 for Entries 1, 2, 7, and 8. The expected ratio for Entries 3 and 4 would be 1 non-nodulated:3 nodulated, and the expected ratio for Entries 5 and 6 would be 1 non-nodulated:15 nodulated. The summed chi-square values, each having 1 df, for Entries 5, 6, 7, and 8 are 62.8, 9.92, 4.42, and 15.15, respectively. Each of these values has a probability of < 0.05. This indicates that the model reported in Chapter 3 describes the mode of inheritance of nodulation for this population better than the model reported by Nigam et al. (39).