Why does it make sense for scientist to test just one variable at a time in an experiment?

The spermists believed that: the male made the only hereditary contribution to the offspring the male and female traits were blended in the offspring the female made the only hereditary contribution to the offspring the male and the female made equal contributions to the next generation

What term describes a doctor who becomes a specialist by meeting standards beyond those of getting a license and passing a test in the specialty area?

Why zebra fish experience similar genetic disease as human