Photo of Eldoncard ABO Rhesus D blood test

I have a question about blood types and seeming hereditary contradictions.

The above image of an Eldoncard blood test, which shows quite clearly that the testee has AB- type blood. (see image!). The sample comes from a person that has a father with blood type A (known from military), and a mother that has blood type O- (known from being a blood donor).

So the question is: how can a person have AB- in this situation (Mother with O-)?

There are a couple of possible scenarios:

  1. The Eldoncard ABO Rhesus D-test could be wrong, and the testee could have another blood type.
  2. The mother could have a weak manifestation of the B subtype, something which would show her as being O- while in fact being bO-, thus having been misdiagnosed as O-
  3. The father could actually have AB blood type, and in some rare circumstances, there have been examples of inheritance of blood type coming from only one parent.

Secondary question, though: If the mother has a weak manifestation of the B subtype, and this is how the testee got one B from her, wouldn't also they also have a weak manifestation of the B subtype - example Ab?

For the purposes of the question, assume that both parents are in fact the genetic parents of the testee.

  • Good Q; recommend for migration to BiologySE as it is primarily about genotype/phenotype inheritance
    – DoctorWhom
    Commented Jan 15, 2018 at 20:21
  • It can be migrated if it is depersonalized, otherwise they will close it as personal medical advice (ironically).
    – JohnP
    Commented Jan 16, 2018 at 15:18
  • Your question fascinates me because I was in a sad quandry over the blood types of my parents' in relation to my own. I knew my mother's blood type and my own to be "true". I had my (still living at 100 yrs old) father's military dog tags from World War Two. Dog tags are engraved with the sailor's blood type. The blood type on the tag revealed that he could not be my biological father. This gave me so much misery for the past several years and I felt compelled to hold the "secret" from everyone I knew. During a discussion on my father's recollections of his Naval training this past Christmas, Commented Jan 18, 2018 at 18:09
  • 2
    ... he laughed that it was a damn good thing he was never wounded because his tags bore the wrong blood type, Case closed. Sometimes mistakes are made. Commented Jan 26, 2018 at 1:30

1 Answer 1


Let's assume that the person inherits their A version of their ABO gene from their father. This means that they inherited their B version of their ABO gene from their mother. Now mother apparently tests O, which means that she can only donate an O gene to her offspring, unless she has a non-functional H gene. People with non-functional H genes will test as O blood type even if they're actually A or B type.

The child then gets a functional H gene from the other parent and this then exposes the A or B gene from the mother.

What actually happens is that the A or B gene code for proteins that turn the H protein into A or B, and if the H gene is defective, the A or B proteins can't be made and so the blood type appears as O. http://genetics.thetech.org/ask/ask413

Another possiblity is a mutation.

enter image description here

But you're much more likely to mutate from O to A rather than to B as the former pair only has one base difference between them. The O type is likely a nonsense mutation from A. http://genetics.thetech.org/ask/ask181

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