You're a genetic counselor. A mother and father both with type AB blood have a child with blood type O. A second type-O child was born six years later. The father is upset and worried that, perhaps, he is not the biological father. After extensive genotyping, you determine that he is most likely the father of both children. This is an example of the "Bombay phenotype". A separate locus codes for a precursor to the A & B antigens (the H antigen). If the child is hh, no H antigen is made and the child will be phenotypically type O. What is the name of the genetic phenomenon that accounts for the Bombay phenotype?