Genotype does not determine phenotype uniquely. The same genotype can produce different phenotypes depending on environmental conditions — this is phenotypic plasticity.

The classic example: a single caterpillar species produces different adult wing patterns depending on the temperature it experienced as a larva. The gene for "what wing pattern to produce" interacts with the environment.

This complicates the nature/nurture framing. It's not genes *vs.* environment but genes *and* environment, interacting in ways that can be context-dependent, reversible, and heritable (via epigenetic mechanisms).