Hi,

I read the documentation, vignette, tuto ... but i'm face an issue.

Let's try :

dds <- makeExampleDESeqDataSet(n=10000,m=12)
dds$condition <- factor( c( rep("Ctrl",6), rep("Trt",6) ) )
dds$genotype <- factor(rep(rep(c("WT","MU"),each=3),2))

dds$genotype = relevel( dds$genotype, "WT")

If i want to modelise the effect of treatment for genotypes:

Méthode A :

design(dds) <- ~ genotype + condition + genotype:condition

res = results(dds, name="genotypeMU.conditionTrt")

that's ok but :

Méthode B

dds$group <- factor(paste0(dds$genotype, dds$condition))
design(dds) <- ~ group

res = results(dds, name="group_WTTrt_vs_MUCtrl") doesn't give the same result

What's i'm wrong ?

Thanks,

Please look at the diagram in the vignette in the section on interactions.

The most recent DESeq2 question on the support site had the exact same question.

Above you are mixing up main effects and interactions. The interaction is not a main effect, but a difference between the two condition effects.