What is it called when 2 AB parents have an O type child?

When two parents with AB blood types have a child with O type blood, it is known as the Bombay phenotype. This rare condition occurs due to a genetic variation that affects the synthesis of the H antigen, a precursor substance necessary for the formation of A and B antigens. As a result, individuals with the Bombay phenotype lack both A and B antigens on their red blood cells, leading to an O blood type.