| Parents' blood types | Possibility | Child's blood type | Impossibility | Child's blood type |
|---|---|---|
| O + O | O | A , B , AB |
| O + A | A , O | B , AB |
| O + B | B , O | A , AB |
| O + AB | A , B | O , AB |
| A + A | A , O | AB , B |
| A + B | AB , A , B , O | |
| A + AB | A , B , AB | O |
| B + B | B , O | A , AB |
| B + AB | A , B , AB | O |
| AB + AB | A , B , AB | O |
ABO blood type is a classification system for blood types. In humans, blood types are classified into four types: A, B, O, and AB. There are three alleles that determine the type: A, B, and O, and six genotypes: AA, BB, AB, AO, BO, and OO.
Thus, blood type is determined by genes, but the information needed for blood transfusions and other such procedures is only four: A, B, AB, and O, and genetic combinations are rarely taken into consideration. However, because blood type genes follow Mendel's law, it is sometimes possible to guess the genetic combination from the blood types of the parents and child. For example, if a couple both have type A and a child has type O, both parents' genetic types will be AO. For a type O child to be born, a sperm carrying an O gene must fertilize an egg carrying an O gene, and if the parent's blood type is type A, then there must be at least one A gene.