Would you ever eat a plant whose genes were altered by scientists in a laboratory? You probably already have, for genetically modified organisms (GMOs), also known as genetically engineered organisms, are ＿＿（16）＿＿ in modern food supplies. The concept underlying genetic modification is not new. For centuries, farmers have used a method called selective breeding to produce more ＿＿（17）＿＿ crops. They do this by choosing seeds from plants that appear to be particularly ＿＿（18）＿＿ to pests and cold or dry weather and planting those seeds in place of weaker strains. Over time, the desirable strains come to dominate the genetic ＿＿（19）＿＿ of the farmers' crops. Not everyone is supportive of the technology, however. Critics point to the fact that GM foods are simply too new to be embraced, ＿＿（20）＿＿ that there could exist long-term health risks that have yet to surface. For instance, consumers with allergies have had adverse reaction to the implanted genes.