Subject: What do developmental biologists mean by ' dominant negative?'

I have frequently heard developmental biologists refer to dominant 
negative mutations or dominant negative forms of a gene or protein.  I 
know from genetics what dominant means and what negative means, but 
the two terms used together don't make sense to me and I can't find a 
reference to such usage in my texts.  I have presently encountered 
this usage in a review by Tanabe and Jessell in Science 274:1115-1123. 
The authors are describing cell type differentiation in the developing 
spinal cord.  They state, "Injection of transcripts that encoded a 
dominant negative form of an activin receptor blocked mesodermal 
differentiation."  Care to comment?

Re: What do developmental biologists mean by ' dominant negative?'