- alleles
- antigen
- base pair
- blood types
- gene
- Genetic deletion
- genetic mutation
- locus (genetics)
- Muller's morphs
- Northern blot
- oligonucleotide
- phenotype
- protein
- RecLOH
- Western blot
- Zygosity
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A mutant allele that produces no protein is called a protein null (shown by western analysis), and one that produces no RNA is called an RNA null (shown by Northern analysis or by DNA sequencing of a deletion allele). A genetic null or amorphic allele has the same phenotype when homozygous as when heterozygous with a deficiency that disrupts the locus in question. A genetic null allele can be a protein and RNA null, but can also express normal levels of a gene product that is non-functional due to mutation.
Another definition of null allele concerning molecular markers, refers to such a marker in the case it can no longer be detected because of a mutation. For example, microsatellites (i.e. a repetitive sequence of DNA, in which the repeat is rather short) are used as molecular markers amplifying them through PCR. To do so, a primer or oligonucleotide aligns with either of ends of the locus. If a mutation occurs in the annealing site, then the marker can no longer be used and the allele is turned into a null allele.
One example of a null allele is the 'O' blood type allele in the human A, B and O blood type system. The alleles for the A-antigen and B-antigen are co-dominant, thus they are both phenotypically expressed if both are present. The allele for O blood type, however, is a mutated version of the allele for the A-antigen, with a single base pair change due to genetic mutation. The protein coded for by the O allele is enzymatically inactive and therefore the O allele is expressed phenotypically in homozygous OO individuals as the lack of any blood antigen. Thus we may consider the allele for the O blood type as a null allele.
See also
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