The "O gene" refers to a specific gene associated with the blood type system in humans. It is part of the ABO blood group system, which includes types A, B, AB, and O. The O gene is responsible for producing a type of sugar molecule that is not present in blood types A and B, making it a universal donor for blood transfusions. Individuals with the O blood type have two copies of the O gene, which means they do not have A or B antigens on the surface of their red blood cells. This characteristic allows them to donate blood to any other blood type, making them crucial in emergency medical situations. Understanding the O gene is important for safe blood transfusions and organ transplants.