If you want to reduce the likelihood of any preexisting differences between the groups of participants assigned to different conditions in an experiment you should use

Random Assignment

Explanation:

Random Assignment

Is to randomly assign subjects to different treatment groups to reduce the chances of preexisting differences between the subjects having an impact (negative) on the experiment.

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the father in the first generation probably had the recessive colorblind trait in his x chromosome. if he and his wife both had the colorblind trait, even if she had 1 recessive and 1 dominant, then that trait would be passed on to their kids.

(since you didn't attach the family tree this is just an assumption)