Blood Types 101: An Introduction to ABO Blood Types and the Rh System

Because blood tests are common many people have questions about the genetic information found in the ABO blood group and Rh system.

Question

If both of my parents have O positive blood, is it possible for me to have O negative blood?
Debbie

Dr. Greene's Answer

Genetics can be so confusing! I can see how the issue may appear murky. Because blood tests are common many people have questions about the genetic information found in the ABO blood group and Rh system.

The modern science of genetics had its start in 1866 when an Austrian monk named Gregor Mendel provided a simple yet powerful description of how traits are passed on from one generation to another. Mendel’s work was unappreciated until 1900, more than fifteen years after his death. In his initial formulation, he described how sexual beings get two genes for each trait, one from each parent. The trait expressed, or visible, is a result of the interplay between these two genes. Specifically, he recognized that some genes are dominant and some are recessive. If you have one copy of a dominant gene you will express that trait, regardless of the other gene. In order to express a recessive trait you must have two recessive genes.

Mendel’s first experiments, though simple, were quite profound. He worked with peas, which had easily distinguishable traits, such as green versus yellow seeds. Each pea has two seed-color genes, one from each parent. The peas with two yellow genes were yellow. Those with a yellow and a green gene were also yellow; only those with two green genes turned out to be to green. Thus yellow was dominant over the recessive green gene.

The situation with human blood genetics is far more complex because at each point there are multiple possible characteristics. Nevertheless, the genetics of human blood is far better understood than that of any other human tissue.

ABO Blood Types

First let’s look at the ABO blood types. Each person receives an A, a B, or an O gene from each parent. In this system, the A and B genes are co-dominant, and the O gene is recessive. Thus 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. This means that a child with type O blood could have parents with type A, type B, or type O blood (but not with type AB). Conversely, if two parents both have type O blood, all their children will have type O blood.

Rh System

Another medically important blood type is described in the Rh system. These genes were first discovered in the rhesus monkey, hence the designation Rh. 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. Thus if your genetic type is ++ or +-, your blood type will be Rh positive. Only if your genetic type is — will you be Rh negative. This means that 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. Children who are Rh negative can have parents who are either Rh positive or Rh negative.

This is why two parents who have O positive blood could easily have a child who is O negative. In fact, most children who are O negative have parents who are positive, since the +- combination is so much more common than the — combination.

Non-ABO, Non-Rh Systems

As it turns out, there are more than a dozen complete blood group systems other than the ABO system and the Rh system. This enables us to look at inheritance and family trees with greater precision.

Specific tests are available (through blood banks) to determine whether someone is a child’s parent.

Unless such testing indicates otherwise, there is no reason, based on your blood type, to be concerned that your parents might not really be your parents.

Last medical review on: March 02, 2008
About the Author
Photo of Alan Greene MD
Dr. Greene is a practicing physician, author, national and international TEDx speaker, and global health advocate. He is a graduate of Princeton University and University of California San Francisco.
Get Dr. Greene's Wellness RecommendationsSignup now to get Dr. Greene's healing philosophy, insight into medical trends, parenting tips, seasonal highlights, and health news delivered to your inbox every month.
Showing
10
comments (52 total)
Add your comment

Recent Comments

Hi Caty,

It looks like your previous comments are on a different thread: Blood Types 102: The Role of A, B, O, and AB Groups In Determining Paternity.

Please let me know if that does the trick.
Best, @MsGreene
Note: I am the co-founder of DrGreene.com, but I am not Dr. Greene and I am not a doctor. Please keep that in mind when reading my comments and replies.

Hi Cheryl
I noticed alot of the questions and answers are not on the page anymore ( hoping im not on the wrong site)especially those of 2018 and including my previous question that you answered in June 2018…. An observation… hoping all is well

Hi Caty,

It looks like your previous comments are on a different thread: Blood Types 102: The Role of A, B, O, and AB Groups In Determining Paternity.

Please let me know if that does the trick.
Best, @MsGreene
Note: I am the co-founder of DrGreene.com, but I am not Dr. Greene and I am not a doctor. Please keep that in mind when reading my comments and replies.

Hi Katie,

You have indeed understood it correctly.

Hope it helps!
Best, @MsGreene
Note: I am the co-founder of DrGreene.com, but I am not Dr. Greene and I am not a doctor. Please keep that in mind when reading my comments and replies.

Thank you so much for your reply, it has helped me understand.
So the + and- part of a blood group is accounting for the D antigen being present or not, it’s just always confused me because I thought rhesus was separate to being just + or – like it referred to something else, a different protein. So I understand that if a person is A- they are in fact rhdA- like myself and if someone is O+ they are rhdO+, they aren’t different blood groups entirely…
Therefore the A is from me and the + is from his father.
I hope I’ve understood it correctly, again thank you for your time, very much appreciated.
Katie

Hi Katie,

You have indeed understood it correctly.

Hope it helps!
Best, @MsGreene
Note: I am the co-founder of DrGreene.com, but I am not Dr. Greene and I am not a doctor. Please keep that in mind when reading my comments and replies.

Hi Katie,

Thanks for writing in.

First, blood types can not be used to determine paternity, but in some cases they can rule out a potential father. In the combination you describe above, it is possible that the man you believe to be the father of your child is the father. If you are A- and if your son’s father is O+ your son could have gotten the A from you and the Rhd+ from his father.

Here is how that would work:
Blood types are comprised of two alleles. The most common (by far) are A, B, and O. A child gets one allele from each parent.
— If a person has type A blood he or she has at least one A allele. He or she can have two A alleles (AA) but can also have one A and one O (AO). Both are called type A blood. That person can hand down an A or an O allele to his or her child.
— If a person has type O blood he or she has two O alleles and can only hand down an O allele.

Blood can either have the D antigen or not have it. Rh+ means a person has the D antigen or is positive for the D antigen. Rh- means a person does not have the D antigen present.
— If a parent is Rh- he or she can not pass down the D antigen. It is not present.
— If a parent is Rh+, he or she can pass down the D antigen.

What all this means for you: If you are A-, and if the father is O+, your child could have gotten the A from you and an O from his father. He would then be type A. He would have gotten the Rh D antigen from his father. As I said, this does not prove paternity, but it does not rule it out.

A DNA paternity test is the only test considered definitive. They are fairly inexpensive, can be ordered online, taken in the privacy of your own home, mailed in, and results received by mail. They are considered 99.999% accurate.

I hope that helps.
Best, @MsGreene
Note: I am the co-founder of DrGreene.com, but I am not Dr. Greene and I am not a doctor. Please keep that in mind when reading my comments and replies.

Thank you so much for your reply, it has helped me understand.
So the + and- part of a blood group is accounting for the D antigen being present or not, it’s just always confused me because I thought rhesus was separate to being just + or – like it referred to something else, a different protein. So I understand that if a person is A- they are in fact rhdA- like myself and if someone is O+ they are rhdO+, they aren’t different blood groups entirely…
Therefore the A is from me and the + is from his father.
I hope I’ve understood it correctly, again thank you for your time, very much appreciated.
Katie

If a mother has AB- and the Father an O+ is it possible for the child to have the recessive O+ blood?

Hello

After reading this article, and quite frankly a million others I’m still confused. My problem is with the rhesus system and I’m hoping you can help put my mind at ease.
I am Rhd A-, my son is Rhd A+, I do not speak to my sons father as he refuses to believe he is his, because, the fatgersblod type is o+ he says it is impossible for my son to be his! I’m so confused with it all, please help me understand.

Hi Katie,

Thanks for writing in.

First, blood types can not be used to determine paternity, but in some cases they can rule out a potential father. In the combination you describe above, it is possible that the man you believe to be the father of your child is the father. If you are A- and if your son’s father is O+ your son could have gotten the A from you and the Rhd+ from his father.

Here is how that would work:
Blood types are comprised of two alleles. The most common (by far) are A, B, and O. A child gets one allele from each parent.
— If a person has type A blood he or she has at least one A allele. He or she can have two A alleles (AA) but can also have one A and one O (AO). Both are called type A blood. That person can hand down an A or an O allele to his or her child.
— If a person has type O blood he or she has two O alleles and can only hand down an O allele.

Blood can either have the D antigen or not have it. Rh+ means a person has the D antigen or is positive for the D antigen. Rh- means a person does not have the D antigen present.
— If a parent is Rh- he or she can not pass down the D antigen. It is not present.
— If a parent is Rh+, he or she can pass down the D antigen.

What all this means for you: If you are A-, and if the father is O+, your child could have gotten the A from you and an O from his father. He would then be type A. He would have gotten the Rh D antigen from his father. As I said, this does not prove paternity, but it does not rule it out.

A DNA paternity test is the only test considered definitive. They are fairly inexpensive, can be ordered online, taken in the privacy of your own home, mailed in, and results received by mail. They are considered 99.999% accurate.

I hope that helps.
Best, @MsGreene
Note: I am the co-founder of DrGreene.com, but I am not Dr. Greene and I am not a doctor. Please keep that in mind when reading my comments and replies.

Hi Barbara,

I can see why you have questions. As you point out if your mother is O and your father is AB, your blood type can not be O. However, your test may not be conclusive and your parents’ blood types may not be what you think they are. Without prying it would be tough to have their blood types confirmed, but you really can’t draw any conclusions with the information you have.

I hope that helps.
Best, @MsGreene
Note: I am the co-founder of DrGreene.com, but I am not Dr. Greene and I am not a doctor. Please keep that in mind when reading my comments and replies.

Hi There,

I know you must be quite sick of these types of questions. I am just trying to get to the bottom of this without prying into my family history to deeply.

My mother has never had any record of my blood group, I recently saw a woman who does “Haemanalysis” from home, she is a medical scientist. I asked her to test my blood group and she did this on those glass swab things from a finger prick. She determined that I was most likely “O” group from the reactions that occurred. However, my father claims to be AB+ and my Mother O+, therefore I couldn’t be his. The plot thickens, I raised this with my mum and she reminded me, I was a test tube baby. Mum had an in rooms procedure where the sperm was placed into her somehow to assist with pregnancy. I don’t look anything like my father nor do I look a thing like my mums fling. help.

Points in the right direction if you can assist at all, would be greatly appreciated.

Thanks

Hi Barbara,

I can see why you have questions. As you point out if your mother is O and your father is AB, your blood type can not be O. However, your test may not be conclusive and your parents’ blood types may not be what you think they are. Without prying it would be tough to have their blood types confirmed, but you really can’t draw any conclusions with the information you have.

I hope that helps.
Best, @MsGreene
Note: I am the co-founder of DrGreene.com, but I am not Dr. Greene and I am not a doctor. Please keep that in mind when reading my comments and replies.

Samira,

Thanks for writing in.

It’s impossible to determine paternity just from blood types, though in some cases it is possible to determine that a person with a specific blood type can not be the father. In this case, if a woman has O- blood and if a man has B- blood they could have a child with B- blood and a child with O- blood.

If you are concerned about paternity, DNA paternity tests can provide the information you’re looking for with 99.9% accuracy.

I hope that helps.
Best, @MsGreene
Note: I am the co-founder of DrGreene.com, but I am not Dr. Greene and I am not a doctor. Please keep that in mind when reading my comments and replies.