Answer: Knock down the gene to see it is sufficient
Knock out the gene see if it is necessary
Explanation:
The expression of the Gene X can be changed by inserting a promoter which can control the expression like lacZ. Then with this promoter effect you can see what happens when the expression of Gene X is lower.
To see its necessity, the gene can be knock down by CRISPR Cas9 or other techniques. You can see the effect when it is not translated at all.
