The "O allele" is one of the three main alleles that determine human blood types, specifically in the ABO blood group system. It is represented by the letter "O" and is considered a recessive allele. This means that an individual must inherit two O alleles (one from each parent) to have blood type O. If an individual has at least one A or B allele, they will have blood type A or B, respectively. Blood type O is significant because it is often referred to as the universal donor type. This means that individuals with blood type O can donate blood to any other blood type without causing an immune reaction. However, they can only receive blood from other type O donors. Understanding the O allele is crucial in fields like transfusion medicine and genetics.