5.9 years ago by
You might have a typo (as @komal.rathi hints at):
"[if] the same gene in D vs. B has log2 = -4 .. then can we estimate that the difference D vs B is..."
You don't have to estimate the difference of D vs. B, you just told us what it is.
On the other hand, if you've done B/A and D/C, and you want to use that data to estimate D/B, you can't do it directly without knowing something about the controls.
You don't mention what technology you're using (Northern Blot? qPCR? RNA Seq?), thus it's hard to assess what assumptions come into play. However if we assume RNA Seq data, and you have large populations of normalized values, you can't simply estimate the D/B difference reliably without examining the A/C difference to ensure that the gene is unchanged in both controls. Otherwise we can assume the data sets are all normalized to each other and the following numbers would satisfy the criteria log2(B(10)/A(40)) = -2, log2(D(10)/C(160)) = -4, yet log2(D/B) != -6 because the gene is changing in each control. But again, this assumes a typo, and we don't know what normalization assumptions you are employing.