Step 1: Understand the question.
We must name the case where one single gene controls more than one trait at the same time.
Step 2: Define the key idea.
When a single gene affects several different traits, often traits that seem unrelated, the effect is called pleiotropy.
Step 3: Give a quick example.
In sickle cell anaemia, one changed gene affects the red blood cells, leading to many problems in the body at once. That is pleiotropy in action.
Step 4: Rule out polygenic inheritance.
Polygenic inheritance is the reverse case. There many genes together control one trait, like skin colour. So it does not fit.
Step 5: Rule out multiple allelism.
Multiple allelism means one gene has more than two alleles in the population, like the ABO blood group gene. That is also not what is asked.
Step 6: Choose the answer.
One gene giving many traits is pleiotropy.
\[ \boxed{\text{Pleiotropy}} \]