I have not been on the web for a while.
When we say a single gene animal, we mean an animal with a single mutant gene in the gene pair. A single gene spider ball python has one spider mutant gene and one normal gene in the gene pair. A single gene pastel ball python has one pastel mutant gene and one normal gene in the gene pair.
As Kutilein wrote, a normal gene does something rather than nothing. A mutant gene either does nothing or does something different from what the normal gene does.
Here is a way to think of it. Norman (normal gene) and Mary (mutant gene) are in a car approaching an intersection. Here are the possibilities.
1. Norman has two hands on the steering wheel. Mary is not in the car. Car always turns left at the crossroads.
2. Norman is not in the car. Mary has two hands on the wheel. Car always turns right at the crossroads.
3. Both Norman and Mary are in the car. Norman has one hand on the steering wheel, and Mary has one hand on the wheel. Which way does the car turn?
3a. Norman has one hand on the steering wheel and Mary has one hand on the wheel. Mary is recessive to Norman. Norman tries to turn left, and Mary tries to turn right. Mary applies much less force to the wheel than Norman does. Car turns left.
3b. Norman has one hand on the steering wheel and Mary has one hand on the wheel. Mary is dominant to Norman. Norman tries to turn left, and Mary tries to turn right. Mary applies much more force to the wheel than Norman does. Car turns right.
3c. Norman has one hand on the steering wheel and Mary has one hand on the wheel. Mary is codominant to Norman. Norman tries to turn left, and Mary tries to turn right. Norman and Mary apply equal force to the wheel. Car goes straight instead of turning.