An amorphic mutation, or amorph, is a genetic mutation resulting in either reduction or absence of the gene product at the molecular level, or absent function at a molecular level, also called a null allele.
In the human ABO blood group system, the alleles for the A-antigen and B-antigen are co-dominant, thus they are both expressed if both are present, giving blood type AB. The allele for O blood type, however, is actually exactly the same as the allele for the A-antigen, but with just one different base pair of neucleotides, probably due to genetic mutation. The protein coded for is non-functional and therefore the gene is expressed phenotypically as the lack of any blood group antigen. The allele for O blood type may therefore be considered as a null allele.