Final answer:
The gene that needs to be present in only one parent to have a 50-50 chance of affecting each child is called the dominant gene.
Step-by-step explanation:
The gene that needs to be present in only one parent to have a 50-50 chance of affecting each child is called the dominant gene. When a parent has one dominant and one recessive factor for a trait, there is a 50% chance that the dominant factor will be passed on to the child.