Bombay blood type
Bom·bay blood type(bom-bā' blŭd tīp)
Blood type of those who possess the genes for A and B antigens but are unable to express the genes because they lack the gene for H antigen, a required precursor of A and B. People with this blood type frequently have anti-H in their blood.
Medical Dictionary for the Health Professions and Nursing © Farlex 2012
Bombay blood typea rare blood type in which individuals appear to be Group O (see ABO BLOOD GROUP) and yet can produce offspring of a type normally impossible for a Group O parent. It appears that Bombay types are homozygous (see HOMOZYGOTE) for a pair of recessive h alleles that inactivates the production of the ABO blood group antigen, even though they have the genes to manufacture these antigens, a form of EPISTASIS.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005