Biological magnification, or biomagnification, occurs when pollutants taken up by organisms at the base of the food chain reach high concentrations in the bodies of animals at the top of the food chain. Effects of biomagnification vary widely depending on the pollutant, organism and ecosystem in question.
1. It allows the scientist to determine the effect of the changed variable.
Let's say there was an experiment showing the effect of water on plant growth. The environment (in this case, the other variables including the amount of sunlight) would need to be kept under control in order for the results to be clear. If there was a different amount of sunlight during each trial, then the results would be effected by both the water and sunlight, making the results incorrect for the experiment.
Homologous, this means they are the same chromosome from each parent matched together