It's more than that disaster waiting to happen. There's a load of scenarios possible, and ALL of them are possible only due to idiots allowing patenting of genes.
- GM Monoculture gets hit by disease: global short-term problem.
- Everything else gets hit by disease manufactured by GM manufacturer. World Domination, but probably suspicious.. There's a short story "The Calorie Man" by Paolo Bacigalupi which describes this.
- GM crops cross pollinating everything else, making it impossible to NOT buy patented GM crops (because everything else will be illegal). Likely but rather slower scenario, will also leave most of the worlds food supply in the hands of GM companies.
I'm pretty sure there's much more of those.
Patents on genes are at least as harmful as the ones on software. And in fact, the whole patent system must be abolished. Such a mercantilistic monopoly-generation-system just has no place in a free market.