You are interested in studying a gene called cftr because mutations in this gene in humans cause cystic fibrosis. you have made a line of mice that lack the mouse cftr gene. these mice are unable to clear bacteria from their lungs, so they get lung disease. you put a normal human cftr gene into some of these mice and discover that the mice with the human gene are able to clear bacteria from their lungs and no longer get lung disease. from this experiment, you can conclude that: