null allele

The inability to detect an individual’s allele during DNA testing. Possible causes include: a variant (mutation) in the template DNA that prevents a PCR primer from binding properly and thus preventing the generation of a DNA product or the loss of the DNA sequence for the allele from the genome due to a deletion. The detection of a single allele may suggest the individual is homozygous when the individual is actually heterozygous at that locus, but the second allele is undetected with the method used.