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Transplant Q&ASee: Chart with possible/impossible blood type combinations Question: My husband and I just completed the ABO blood typing for a possible living-related liver transplant for our child. We were shocked to learn that while we are both blood type B, our child has type O blood. How is this possible?Answer: To understand this, you need to understand blood type genetics. There are four basic blood types, in order of frequency from most common to rarest they are: O, A, B, and AB. Blood type is determined by the "alleles" that we inherit from our parents. Alleles are possible types of a particular gene, in this case the blood type gene. There are three basic blood type alleles: A, B, and O. We all have two alleles, one inherited from each parent. The possible combinations of the three alleles are:
Blood types A and B are called "codominant" alleles, while O is "recessive." A codominant allele is apparent even if only one is present; a recessive allele is apparent only if two recessive alleles are present. Since blood type O is recessive, it is not apparent if the person inherits an A or B allele along with it. So, the possible allele combinations result in a particular blood type in this way:
You can see that a person with blood type B may have a B and an O allele, or they may have two B alleles. If both parents are blood type B and both have a B and a recessive O, then their children will either be BB (if each parent passed on the B allele), BO (if one parent passed on B and the other parent passed on O), or OO (if both parents passed on the O allele). If the child is BB or BO, they have blood type B. If the child is OO, he or she will have blood type O. When you understand blood type genetics, you can see that it is not
at all unusual for two parents with blood type B (or blood type A) to
have children with blood type O.
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Children’s Liver Association for Support Services
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