Why would DNA react this way? If you look for the logic in this decision, you might see it like this: by smoking during pregnancy, you are telling the embryo that the air is full of toxins and that breathing is sometimes dangerous. The developing lungs would do well to be able to react quickly to any inhaled irritants. Asthmatic lungs are over-reactive. They cough and spit at the slightest whiff of foreign aerosols. Still, I believe even a genome as abused as this can be reminded of normal function.