How Are We Born With Our Blood Types?

I am trying to understand blood types and how we get the type of blood we have. I am confused. If a person with type-O blood can donate blood to a person with type A and be compatible, why then can’t two people with type-O blood produce a baby with type-A blood. Or can they?

As I understand it, type “O” means that you have neither the “A” nor “B”: that out of the two genes you have that code for these proteins, you have neither. Therefore, it would be impossible to pass on the gene for type “A”. Blood types are codominant not recessive, so it couldn’t come from anywhere other than parents (not a generation skipping type of thing) — am I wrong? If not, what about the donation question?

I have rarely received so many questions and comments about one of my postings as I have about the ABO blood groups, their inheritance, and how they affect transfusions, so I’m going to try again.

About 20 different blood groups have been discovered, but the ABO group is the best known, and along with the Rh group the most significant for transfusions.

Like most inherited traits, the ABO group is transmitted to each individual by genes, one gene received from each parent. (The genes for some other traits are inherited only from the mother, but that’s another story.) As you point out, the genes determining the ABO type are codominant, meaning that an A from one parent does not take precedence over a B from the other. The gene for O creates a protein that is inactive; so if an O from one parent is paired with an A from the other parent, the person’s blood type will be A. Possible blood types in the ABO system are therefore A, B, AB, and O.

If we look at the two genes that people with each blood type carry, we see several possibilities.

Possible gene combinations

  • AA or AO results in A
  • BB or BO results in B
  • AB results in AB
  • OO results in O

When considering the inheritance of the types, it is more useful to look at the genes the person is carrying than at their actual blood type. Since sperm and eggs carry only one gene, someone with AA genes (type-A blood) will make sperm or eggs that all carry one A gene. Someone with AO genes (type-A blood also) will make sperm or eggs which are roughly half A, and half O. Someone with OO genes (type-O blood) will make only sperm or eggs carrying O.

Therefore two type-O people, each of whom can make only O eggs or sperm, can produce only children who have OO genes, and therefore must have type-O blood.

If two AA people have children, then again, the children must have AA genes and be type A. But when two AO people — who both test as type A just like the AA people — have children, the situation is more complicated. Roughly one-quarter of the children will be AA, one-half will be AO, and one-quarter will be OO. Both the AA and AO children will have type-A blood when tested, so three-quarters of the children will test as type A, and one-quarter will test as type O. (The fractions are statistically accurate, but may vary in actual people)

The transfusion problem arises because all people carry antibodies against the ABO genes that they don’t possess. In the recipient of a transfusion, these antibodies do the damage when we have a transfusion reaction. A person with type-O blood has antibodies against A and B, so if that person receives a unit of type-A blood by accident, the transfused blood will agglutinate — it’s actually like curdling — and will produce the reaction, which is often fatal. The type-B person has antibodies against the A gene, and vice versa. The type AB individual doesn’t have antibodies against any of the genes, and so theoretically can receive blood from any other person. Therefore, type-AB people are called “universal recipients”. Type-O people are termed “universal donors,” since their blood cells carry no A or B genes and will therefore not be agglutinated by the antibodies of the person receiving the transfusion. In practice, transfusions of blood of dissimilar types are rarely given, but in wartime or in emergencies type-O blood has been given as the universal donor to people with other blood types.

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