I don't believe this question is suited for Biology.SE, Medicine.SE, or any other stack, since it involves a real mechanism which is not applied in real life. And not every question has to be about magic.

I would like to know if, by modifying their genes, do humans have a 100% chance of receiving those traits, or does it just increase the chance of getting them?

Say we modify a child with the genes to be really smart, will the child 100% be smarter than people without the gene, or will he just have a higher chance to be smarter?

Physically, does giving somebody the blue-eyes gene makes them automatically have blue eyes, or do they just get an increased chance of having blue eyes?