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 group. In a close relative's case, while her parents and two siblings have O+ as their blood group, she ended up having A+. We are puzzled as to 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).
