It's common to think of traits as being determined entirely by genes, but in reality, the environment plays a huge role in gene expression. A single genotype can result in multiple different phenotypes due to environmental factors.

The reason for this is the gene is maximally active from 15-25°C and inactive above 35°C. Think about what parts of your body get cold first when you're out in the winter - it's usually extremities like your fingers, nose, ears, etc.!

So what happens is the colder parts of the rabbit produce the protein still, while the warmer have the protein denature, preventing pigment formation.

In humans, the gold standard for studying gene-environment interactions are "twin studies". Identical twins (monozygotic) have identical DNA, but when raised in different environments, can develop some noticeable differences. By studying identical twins separated at birth, it can give great insights into the role that genes and genetics have.

Many traits, especially in humans, rely on both genes and the environment and are not quite as simple as purely genetic, Mendelian traits. For example, someone might have an allele that makes them more likely to have type 2 diabetes, by lifestyle facotrs such as diet and exercise play a huge role in whether someone as the disease or not. Likewise, a plant might have genes for tall growth, but if grown in nutrient-poor soil, it won't reach its full height potential.