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Genetics question

1. In peas, the chemical pisatin is associated with defense against parasitic fungi: normal plants are resistant to fungi and contain pisatin. Two pure lines were obtained, both of which lacked pisatin and were highly susceptible to fungal attack Line 1 was from California and line 2 was from Sweden. The lines were investigated as follows. (Note that the normal lines were also pure breeding.)

(see attachment for chart)

a. Propose a model that explains the results of these crosses. Make sure that you precisely define any symbols that you use.
b. Show the genotypes underlying the parents, F1, and F2 in each cross
c. How do lines 1 and 2 differ with respect to the genetic cause of their lack of pisatin?