How frequent is cis-AB?

Since I have always suspected that this number would be much higher than expected, I am happy to now see proof of that.

The frequency of the cis-AB blood type in blood donors in southwestern Korea.

BACKGROUND: Cis-AB is a rare ABO blood type with an unusual inheritance, on the same chromosome that results from a point mutation. It is relatively common in Korean and Japanese populations. Although more than 100 cases of the cis-AB blood type have been reported, there is no report on the frequency of the cis-AB blood type in Korea. Therefore, we analyzed the frequency of the cis-AB blood type in blood donors in southwestern Korea. METHODS: This study examined 111,842 samples obtained from blood donors recruited at the Gwangju-Chonnam Red Cross Blood Center between July 2002 and Oct 2002. ABO serologic tests were performed on all of the samples and ABO genotyping using the polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method was performed on peripheral blood DNA samples from 129 cases: 24 A2B3, 81 A2B, 23 A1B3, and 1 A1Bx or el case. RESULTS: The frequency of the cis-AB blood type was 0.0331%(37/111,842). Of the 37 cases, 24 cases were the cis-AB/O genotype with the A2B3 phenotype, 9 cases were the cis-AB/A genotype with the A2B phenotype, 3 cases were the cis-AB/B genotype with the A1B3 phenotype, and 1 case was the cis-AB/A genotype with the A1Bx or el phenotype. In the 129 cases, the cis-AB ratio was 100% (24/24), 39.1% (9/23), 3.7% (3/81), and 100% (1/1) in the A2B3, A1B3, A2B, and A1Bx or el phenotypes, respectively. CONCLUSIONS: The frequency of the cis-AB blood type in blood donors in southwestern Korea is about 0.0331%. The most common type of cis-AB was cis-AB/O with the A2B3 phenotype. In addition, one case of cis-AB/A with an unusual A1Bx or el phenotype was found.

So if one of your parents is AB, yet you are O, there is not that much concern.