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During the meiosis of a trisomic cell, when any two homologous chromosomes separate, the third chromosome moves randomly towards any pole of the cell. Trisomic maize generally produces two types of gametes during meiosis: one is the n+1 type, which contains two of the same homologous chromosomes; the other is the n type, which contains one of the same homologous chromosomes. The n+1 type gametes can normally participate in fertilization if they are egg cells, but cannot if they are pollen.  
(1) It is known that resistance to disease (B) is dominant over susceptibility to disease (b). Using trisomic maize with the genotype bbb as the female parent and normal maize with the genotype BB as the male parent for crossbreeding, the genotypes of F<sub>1</sub> are . If F<sub>1</sub> trisomic maize is taken as the female parent and crossed with susceptible normal maize as the male parent, then the ratio of disease-resistant to susceptible offspring is .  
(2) Now, if there is trisomic maize that is susceptible to disease with chromosome 2 being trisomic, and an experiment is conducted to determine whether the susceptibility gene is located on chromosome 2 or another chromosome, then:  
① The experimental approach:  
a. Use homozygous disease-resistant normal maize as the male parent and cross it with the trisomic susceptible maize as the female parent;  
b. Select trisomic plants from F<sub>1</sub> as the ("male" or "female") parent and cross them with susceptible normal maize;  
c. Analyze the ratio of disease-resistant to susceptible phenotypes in F<sub>2</sub>.  
② Experimental results and conclusion:  
If the phenotype and ratio in F<sub>2</sub> are disease-resistant: susceptible = , then the susceptibility gene is on another chromosome;  
If the phenotype and ratio in F<sub>2</sub> are disease-resistant: susceptible = , then the susceptibility gene is on chromosome 2.

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