Bed bugs might sound like an old-fashioned problem, but now they are back with a vengeance. Fifty years ago, the blood-sucking pests were nearly eradicated in the United States thanks in part to the use of pesticides like DDT. Today, they are creeping over sheets and tormenting sleepers across the country. New York was recently declared America's most bed-bug-infested city in America. In the past, we countered the bed bugs by using pesticides called pyrethrins and pyrethroids. These compounds work by attacking the nervous system. Insects have channels in the membranes of their nerve cells that can be opened to allow sodium into the cells, triggering a nerve impulse. Pyrethrins and pyrethroids alter the nervous system by binding to the sodium channels, locking them in the open position. This allows sodium to pour into the cell continuously, causing the nerve to fire repeatedly and eventually leading to paralysis. Todays bed bugs seem to be more resistant to the chemicals. Biologists have determined that mutations are responsible for this resistance. Mutations changing just two of the 2000 amino acids that make up part of the sodium channels would be enough to make an insect 250 times more resistant to a commonly used pyrethroid. These mutations may change the channel so that the insecticide can no longer bind to it effectively. Explain how a mutation causing resistance to pyrethrins and pyrethroids would spread through a population of bed bugs that are being treated with the pesticide. A) In a population of bed bugs, variation exists. Some variation provides resistance to the pesticides. Those bed bugs survive to reproduce, passing the resistance on to offspring. B) Mutations happen randomly to change the gene pool of a population. The mutations are favored when a population of organisms winds up in an environment in which the mutations happen to be useful. C) When a population of insects, like bed bugs, is subjected to pesticides, some of the bed bugs alter their genetic make-up and they are able to survive. They then pass the new genes on to offspring. D) In any population of organisms, sexual reproduction provides for variation in the gene pool. Such variation allows some insects to be resistant to pesticides. When sexual reproduction occurs again, the resistance is passed to offspring.