It's not in the other half of the genome. All cells in an organism share the same DNA, but different cells develop in a very differentiated manner depending on the cells around them (e.g. should it be a neuron or a connecting tissue?). The "decompression algorithm" is in the laws of physics, biochemistry, and the way life works in general. Since we don't understand these well enough, we can't make reasonable information-theoretic claims about the "decompressed" genome.