Null allele

From Wikipedia, the free encyclopedia

A null allele is an allele with the effect of either absence of the gene product at the molecular level, or the absence of function at the phenotypic level.

One example is human blood types A, B and O. The alleles for the A-antigen and B-antigen are codominant, thus they are both expressed if both are present. 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, possibly due to genetic mutation. The protein coded for is non-functional and therefore the gene is expressed phenotypically as the lack of any blood antigen. Thus we may consider the allele for the O blood type as a null allele.