This formulation has appealing properties such as no change being equal to zero a 100 increase is equal to 1 and a 100 decrease is equal to 1. To make this leveled we use log2 for expressing the fold change. Remember that if you use log transformed values you calculate log fold changes using meanY-meanX rather than meanYmeanX.
075 or a drop of 25 from wild type is reported as either 13 fold down or -13 fold change.
You cannot calculate fold change for each sample. But when it is other way round ie treatment 50 control 100 the value of fold change will be 05 all underexpressed genes will have values between 0 to 1 while overexpressed genes will have values from 1 to infinity. There is an alternative definition of fold change citation needed although this has generally fallen out of use. Fold change value represents the ratio between your treated sample and your control sample.