Siamese cats are characterized by light and dark regions of fur. The darker parts occur on the cooler parts of the cat's body because the enzyme that catalyzes the formation of the dark pigment is more active when cool. This example shows that enzymes are affected by .

The correct answer is : temperature.

Explanation:

Siamese cats are known and characterized on the basis of the regions that are light or dark on the part of the body of these cats as according the cooler part of the body.

It is influenced by the enzyme that is active due to low temperature that leads to the formation of the dark pigment in the cooler area and in the part of the body that are higher in temperature are lighter due to inactivity of the enzyme.

Thus, the correct answer is : temperature.

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