The Bombay phenotype is inherited recessively. A person has it only when they receive a non-working copy of the FUT1 gene from each parent. Parents who each carry one copy usually have entirely ordinary blood groups and no reason to suspect anything.
What FUT1 and FUT2 do
FUT1 builds the H antigen on red cells. A closely related gene, FUT2, governs whether the same antigens appear in saliva and other secretions — which is why a saliva test forms part of confirming the phenotype. In the classical Bombay phenotype both are silenced, so the antigens are absent from cells and secretions alike.
Where FUT1 is altered rather than silent, or where FUT2 still works, the result is the para-Bombay phenotype: traces of H antigen rather than none.
Why families matter
Because the inheritance is recessive and both parents carry it, siblings and close relatives are far more likely than the general population to share the phenotype. When someone is identified, screening their family is often the quickest way to find a compatible donor — and to warn relatives who may be carrying it unknowingly.
It is also why registries exist. Finding a match is a search problem, and searching works far better before it is urgent.