[1] A mutant allele that produces no RNA transcript is called an RNA null (shown by Northern blotting or by DNA sequencing of a deletion allele), and one that produces no protein is called a protein null (shown by Western blotting).
A genetic null or amorphic allele has the same phenotype when homozygous as when heterozygous with a deficiency that disrupts the locus in question.
[5] Primers that are specific to a particular locus are used in PCR amplification to bind to these nucleotide sequence repeats and are used as genetic markers.
Divergence from the reference sequences (from genetic mutations) results in poor annealing of the primers so that the marker cannot be used, representative of a null allele.
[7] Null alleles or genes have been studied in different organisms from the red pines of Minnesota to Drosophila melanogaster and mice.