Based on my understanding of Blood group tables, such as the one below a child can only get O blood group when both parents have O as their blood groups. In a close relative's case, while her parents and two siblings have O+ as their blood groups, she ended up having A+. We are puzzled as how this could happen?

Parent-child Blood Group Chart

  • 2
    Blood ABO typing is not as simple as shutterstock makes it. There are weak A types etc. ncbi.nlm.nih.gov/pmc/articles/PMC6910028
    – Fizz
    Dec 2, 2021 at 13:54
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    A much more common occurrence than rare blood groups is non-paternity. Use care when suggesting this to relatives.
    – Ian Campbell
    Dec 2, 2021 at 21:14
  • Someone needs to point out that one possible explanation is that the father isn't who they think it is.
    – Carey Gregory
    Dec 2, 2021 at 22:20
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    I am completely sure that the OP, the relative who got the blood test result, and absolutely everyone they have discussed it with has considered (assumed) that paternity is the issue. I expect that is why OP is asking, to see if there is any other explanation. Dec 3, 2021 at 12:55

2 Answers 2


Although such cases are undoubtedly rare, they are not impossible. Here's a case report from 2005:

Apparent deviation from Mendelian rules of blood group inheritance is rarely observed. Blood group O parents with children expressing weak A subgroups have occasionally been described but not explained. A detailed serological investigation of such a family is described here. [...]

The propositus' RBCs were very weakly agglutinated with monoclonal anti-A but distinctly with polyclonal anti-A,B, i.e. typical for Ax. Serum anti-A1 (titre 4) and -B were present. Her parents' blood groups were both clearly O, with titres of serum anti-A1, and -A at 16 and 4, respectively. Adsorption/elution studies demonstrated A antigen on the daughter's cells only. The ABO genotypes were: mother, AxO1; father, O1vO2; and propositus, AxO2. The Ax allele was an A1-O1v hybrid allele with a crossing-over breakpoint between positions 235 and 446 in intron 6 (Ax-4). Compared to the A1 glycosyltransferase, this allele predicts a protein with two amino acid substitutions (Phe216Ile and Met277Val) known to yield either weakly expressed or no A antigen on RBCs.

This study suggests that the nature of the ABO allele in trans can influence A antigen expression, a phenomenon previously described as allelic enhancement (or reinforcement).


Theoretically, if both parents are O+, there is no chance of an A+ child. Type O blood is recessive, and must be homozygous to exhibit as the blood type. However, if one parent was mistyped, and carried O but had type A blood as his/her dominant type, then of course this would be possible.

Allegedly there are occasional genetic mysteries called "throwbacks" (cf. "atavism") which present unusual anomalies (e.g. two Caucasian parents having a black child). But aside from something of this nature, of whose possibility in this case I am uncertain, some of the possible causes might include:

  • One parent actually has type A blood (could have had blood mistyped, or parent was misidentified).
  • The child was mistyped, and needs to be retested.
  • Blood samples on test day were cross-labeled (mixed up).
  • The parents brought home the wrong infant from the hospital.
  • Etc.

Was the blood test done at school and the child, too afraid of the finger prick, begged some friend to "help"? There are a number of possibilities that one might consider, but to have an A+ child from two O+ parents is not one that I would consider.

Neither does the Red Cross, though they do seem hesitant to make a perfectly definitive declaration on account of potential issues with testing, as seen in their remarks below their blood-type graphic (available HERE).

enter image description here

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    Can you please provide citations to back your claims? Dec 2, 2021 at 13:31

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