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?
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.
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).