Explain why it is important to only change one variable in an experiment and keep everything else constant

In familial hypercholesterolemia individuals homozygous for the allele causing the disorder completely lack receptors on liver cells that take up cholesterol from the blood stream. heterozygous have one-half the number of receptors while individuals homozygous for the normal allele are phenotypically normal. this is an example of a.epistasis b.complete dominance c. incomplete dominance d. codominance

The influenza virus cause the flu, but not other diseases. why?

Ascientific idea becomes valid when it is what?