Null alleles are thought to result from mutations in the
primer sites such that a primer no longer binds to its complement
(Callen
et al
. 1993). If the incidence of null alleles
is high enough, there is a significant likelihood that null
homozygotes would have been observed in a study this
large. Most instances of nonamplification may have been
due to tissue degradation, DNA degradation, random PCR
failure or other artefacts, and were abandoned after two or
more attempts.