Studying one trait at a time ensures that multiple

are not changing simultaneously.

In an experiment usually, two groups are used  

1) Control Group  

2) Experimental Group

During the research, one trait variation at a time is studied in the experimental group in comparison to the control group.

For example:

If we want to study drought stress on plants we take plants group called control, which will be grown under normal conditions while we take other plant groups called the experimental group, which will be grown under all normal conditions except water. Water is given in the reduced form. Both these groups at the end of the experiment are compared with each other. Variation measured will be due to a single trait that is genes related to water stress.

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