上位作用(epistatic effect)：两对基因同时控制一个单位性状发育，其中一对基因对另一对基因的表现具有遮盖作用，这种基因互作类型称为上位作用。
绿色对白色有三对基因差异，其中绿色基因有显性上位作用，只需一个绿色基因G存在，即可抑制淡黄色与白色性状的表达。
GF vs WF had difference in three genes, and the G gene had epistatic effect, one W was enough to restrain the express of YF and WF.
这称为上位相互作用。
上位效应平方和占总产仔数和活产仔数总遗传变异平方和的比例分别为6 3%和6 5 % ，可见上位效应对窝产仔数群体间遗传差异作用不大。
Epistatic component accounted for 6 3% for the total number born, and 6 5% for the number born alive, which showed that epistatic effect was not important in the genetic variation between populations.
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