Are they really my parents?

Question

If both of my parents have O positive blood, is it possible for me to have O negative blood? I have been told that it is not possible, that one parent would have to have the negative factor. I have been researching this question for some time with no good answer. I would really appreciate your help in answering this.
Debbie Howlett - Farmington, New Mexico

Dr. Greene's Answer

Genetics can be so confusing! I can easily see how after much research the issue about your parents would still appear murky.

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, since 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. While there are rare exceptions, the following information on blood types applies to most people.

First, let’s look at the ABO blood types. Each person receives an A, B, or 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 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.

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. 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.

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.

As it turns out, there are more than a dozen complete blood group systems other than the ABO system and the Rh system. This makes great precision possible in looking at inheritance and family trees.

There is no reason, based on your blood type, to suspect that your parents might not really be your parents. If questions linger for other reasons, specific tests are available through blood banks and private laboratories that can settle the issue. These tests are commonly known as paternity and maternity DNA testing (for father and mother respectively). Given the billions of unique people on this planet, the power and precision of genetic testing are amazing.

Last medical review on: April 15, 2009
About the Author
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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.
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Recent Comments

Good Article. Your math is a little off in the beginning. The Australian started genetics in 1866 but was underappreciated until 1900 more than 50 years after his death? He would have had to publish and die no later than 1850 and then be appreciated for the work in 1900 for it to be 50 years after his death. Yet you state he started genetics in 1866?

Hi Liabelle,

Likely one of the blood types is different than you think it is as our current understanding of genetics is that it is not possible for one parent to have O+ blood, the other parent to be O- and their child to have A+. Two parents with type O blood can only have children with type O blood.

As a next step, it is wise to have all your blood retested.

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 Michelle,

Our current understanding of genetics is that it is not possible for one parent to have O+ blood, the other parent to have O- blood and their child to have A+ blood. Two parents with type O blood can only have a child with type O blood.

However, it’s not uncommon for blood types to be different than they are thought. As the next step, it is wise to have all your blood retested.

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.

Hello i was wondering if my child father is o postive and im 0 negative my son is A POSTIVE WHAT DOES THAT MEAN WOULD THAT BE HIS FATHER

Hi Michelle,

Our current understanding of genetics is that it is not possible for one parent to have O+ blood, the other parent to have O- blood and their child to have A+ blood. Two parents with type O blood can only have a child with type O blood.

However, it’s not uncommon for blood types to be different than they are thought. As the next step, it is wise to have all your blood retested.

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.

Hello. My mom Rh+O and dad rh-O and my sister rh+O but how is it possible that I’m rh+A ?

Hi Liabelle,

Likely one of the blood types is different than you think it is as our current understanding of genetics is that it is not possible for one parent to have O+ blood, the other parent to be O- and their child to have A+. Two parents with type O blood can only have children with type O blood.

As a next step, it is wise to have all your blood retested.

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.

Hello Kofi,

Great question. Dr. Greene explains this in-depth in this article — How To Determine and Manage Rh Incompatibility.

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.

I am O-{man} and my will to be partner is 0+. are we compactible to marry, with no consequences afterwards? I need you to explain what may happen if our child inherits my genes ,that is , O-.Thank you.

Hello Kofi,

Great question. Dr. Greene explains this in-depth in this article — How To Determine and Manage Rh Incompatibility.

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 Tejaswini,

There is no need to fear, but it would be wise to have all your blood types retested. Likely one of them is different that you think it is. But if they all come back as you’ve reported, the next step is to have a DNA paternity test run.

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 ,
I have B+ve and my husband has 0+ve blood group , i gave birth to AB+ve Child.. kindly suggest how can my Son have AB+ve ?? i am scared.

Hi Tejaswini,

There is no need to fear, but it would be wise to have all your blood types retested. Likely one of them is different that you think it is. But if they all come back as you’ve reported, the next step is to have a DNA paternity test run.

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 Porshia,

Thanks for writing in. Let me begin by saying that often blood types are different than they are thought to be.

Our current understanding of genetics is that it is not possible for one parent to have type B+ blood, the other parent to have type O+ blood and their child to have type AB+ blood. If one parent has type B+ blood and the other has type O+ blood, each of their children could have blood type B+, or B-, or O+, or O-.

Before you jump to any conclusions, it would be wise to have all your blood retested.

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.