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. 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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  2. 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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  3. 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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  4. 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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