Like humans, flies are attracted to alcohol. Fruit flies (Drosophila melanogaster, above) prefer to lay their eggs on rotten food that can contain ethanol in as high as 7% concentration. (That’s 14 proof to you bar hoppers.) And just like people, the insects differ in their ability to hold their drinks. Biologists know that compared with flies from tropical Africa, flies from temperate regions such as Europe survive longer when exposed to ethanol vapors of high concentrations, and they know it has something to do with enzymes on the flies’ second chromosomes, which break down alcohol and are more active in European flies. But now, biologist James Fry of the University of Rochester in New York has pinpointed a missing piece of the story: the role played by the flies’ third chromosomes. After studying flies collected from Vienna and Cameroon, Fry found that the Vienna flies break down alcohol much faster than Cameroon ones, as expected. But when he replaced the third chromosomes in Cameroon flies with those from Vienna, the African flies gained much more resistance, Fry reports online today in The Journal of Experimental Biology. In a specialized population of flies that could not detoxify alcohol, however, the genetic engineering made no difference whatsoever. Fry suggests that’s because the third chromosomes in European flies help them tolerate acetic acid, a byproduct of internal alcohol breakdown that also gives vinegar its sour taste. There’s no telling what the acetic acid does to the flies, but previous studies on mice have found that it may be responsible for hangover headaches, Fry says.