Understanding Blood Types

Understanding Blood Types

Human blood genetics are quite complex because at each point there are a number of possible characteristics. Nevertheless, understanding blood types, and the genetics of human blood is far easier than that of any other human tissue. To make things a little easier to understand, here are some guidelines for understanding how both the ABO blood types and the Rh system work.

The ABO Blood Types

  • Each person receives an A, a B, or an O gene from each parent.
  • The A and B genes are co-dominant, and the O gene is recessive.
  • A person whose genetic type is either AA or AO will have blood type A, those with genetic type BB or BO will have blood type B, and only those with genetic type OO will have blood type O.
  • A child with type O blood can have parents with type A, type B, or type O blood, but not type AB. Conversely, if two parents both have type O blood, all their children will have type O blood.

The Rh System

The Rh system is actually far more complex than the ABO system in that there are 35 different possibilities that one could inherit from each parent. These, however, are roughly grouped into positive and negative types. In this system the positive are dominant over the negative.

  • If your genetic type is ++ or +-, your blood type will be Rh positive.
  • Only if your genetic type is — will you be Rh negative.
  • If both parents have Rh+ blood with the +- genes, they could have children who are ++, +-, or –. In other words, their children could be either Rh positive or Rh negative.
  • Most children who are O negative have parents who are positive, since the +- combination is so much more common than the — combination.

Dr. Alan Greene

Dr. Greene is the founder of DrGreene.com (cited by the AMA as “the pioneer physician Web site”), a practicing pediatrician, father of four, & author of Raising Baby Green & Feeding Baby Green. He appears frequently in the media including such venues as the The New York Times, the TODAY Show, Good Morning America, & the Dr. Oz Show.

  1. Shayista

    Hello i have just give birth to a baby girl and her blood group is o negative and my blood group is b positive and my husband’s blood group is ab positive. Will blood incompatibility effect my baby im very concern please help

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  2. Sarah English

    I’m o negative
    My son is o negative
    What are the chances of his father having a different blood type?? And what are the possibilities of the blood types his dad could be?

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    • If one parent is O- and the child is O-, the other parent could be A-, B- or O-.

      Hope that helps,
      @MsGreene

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  3. roo

    i’m a negative and my wife is b positive can we have babies

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  4. Mr E

    I am o positive and my wife is o positive but our baby is B positive why?

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  5. sara

    I am rh a negative, my husband is o positive. My daughter is ab positive. Is this a possible combination.

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  6. Crystal L

    I am B+ and my husband is A+ and my son is O- hospital told us this isn’t possible why?

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  7. Linda

    The father of my son is blood type B+
    I am type O.
    My son is A+

    We have had 3 DNA tests and all indicate the father is not his father.
    I have NO reason to believe this. Please HELP with a reply to my email and here also.
    Thank you!

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    • Only you, as the mother, know if no one else can possibly be the father. There are extremely rare genetic mutations that result in unlikely blood type combinations, but, my understanding is our current scientific knowledge and testing isn’t able to determine when those rare mutations actually happen. This puts you in a tough spot. My heart goes out to you and your family.

      @MsGreene

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      • Crystal L

        So are you telling me this is true?

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        • I am telling you that currently our science says if these blood types are all correct, these two people can’t be the parents of this child. BUT new science is developed and in the future we may know something we don’t know now. Sorry I couldn’t be more helpful.

          Best,
          @MsGreene

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  8. Gary

    If my dad is rh neg and my mum is ab neg my sister is o.+me and my other sister are rh o neg.has my sister got same dad as us.

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  9. love

    can DNA say 100 percent the biological father of a baby?

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    • DNA paternity tests are exceptionally accurate. If the father is an identical twin, DNA tests won’t be able to tell which man is the father. Also, there are levels of DNA tests. A complete genome (rarely done) is very, very accurate. If you have a paternity test done, the geneticist who provides the results will be able to tell you the accuracy rating of the test they perform.

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  10. rissa

    My mum is o type blood and my dad is b. My brother is b and my sister is o type. I’m a+ can they still be my parents.

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  11. keykey

    I’m O+ and my boyfriend is B-and ours son is O+ is it possible

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  12. Tracey Murray

    I am A RH Positive blood my mum is o+,my sister is 0_ .Can she still be my mum? What should father be?

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  13. Tasha

    I’m o positive and my husbands A positive … My child is o positive can this be right :/

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    • Your child has a 75% chance of having A type blood and a 25% chance of having O type blood. In addition, your child has a 93.75% chance of having Rh+ blood, so yes, this can be right.

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  14. cherry

    Both my parents are a positive and my brothers are a positive but I’m o negative is this possible?

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  15. Diana

    Hello I have 2 brothers and 2 Sisters they are all A+ and I am the only one who is B+ is it possible for us to have the same biological father?

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    • Mrs. G

      Yes. There are at least two scenarios that immediately come to mind that would account for that. You could have one parent with type O and the other with type AB, or one parent with type A and one parent with type B.

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