一句话知识点：基因上下游关系确定

Q：怎么用双突变体确认两个基因的上下游关系？
A：双突变体（A+B）为单突变体A的表型，则基因B在上游。总开关是下游。

起因：读文献有一句话我不能理解

"pif3-1 ebf1-1 double mutant behaved like pif3-1 single mutant in terms of freezing tolerance and ion leakage. The expression of CBFs and their target genes in ebf1-1 was partially rescued by the pif3-1 mutant either after cold treatment or during the nighttime at warm temperatures. These results demonstrate the PIF3 acts downstream of EBF1 to regulate the CBF pathway and freezing tolerance.

文献涞源：PIF3 is a negative regulator of the CBF pathway and freezing tolerance in Arabidopsis