Each biological parent donates one of their two ABO alleles to their child. A mother who is blood type O can only pass an O allele to her son or daughter. A father who is blood type AB could pass either an A or a B allele to his son or daughter. While a child could have the same blood type as one of his/her parents, it doesn't always happen that way. For example, parents with AB and O blood types can either have children with blood type A or blood type B.
