Allele Drop Out

Allele drop-out occurs when a sample is typed and one or more alleles are not present.

This can be due to a variety of factors:

The initial input quantity of DNA is too low, resulting in the failure to amplify one or more alleles in the sample.
A mutation in the primer binding site is present, which causes a failure in the amplification of the allele.
An allele sizes outside of the normal calling range for a particular locus and goes undetected.