Genetic mechanism of cis-AB inheritance. I. A case associated with unequal chromosomal crossing over

In contradiction to the general Mendelian inheritance of blood group ABO expression, the A and B characteristics are inherited together from one parent in the rare Cis-AB phenotype. Since the synthesis of blood group A and B substances are controlled by N-acetylgalactosoaminyltransferase (A-enzyme) and galactosyltransferase (B-enzyme), the genetic mechanism of Cis-AB expression may be elucidated by examining the characteristics of A- and B-enzymes in Cis-AB plasma. Biochemical study reveals that the examined Cis-AB plasma contains two separable enzyme components: one with kinetic properties similar to those of common A2-enzyme, but differing from A1-enzyme, and another with kinetic characteristics similar to those of common B-enzyme. Therefore, Cis-AB expression, at least in the case examined, is due to unequal crossing over, producing a chromosome with alleles for A2- and B-enzymes, rather than to a structural mutation in A or B alleles producing a single abnormal enzyme with bifunctional activity.