A null allele is a nonfunctional allele (a variant of a gene) caused by a genetic mutation. Such mutations can cause a complete lack of production of the associated gene product or a product that does not function properly; in either case, the allele may be considered nonfunctional. The presence of a null allele cannot be distinguished from deletion of the entire locus solely from phenotypic observation.
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. A genetic null allele may be both a protein null and an RNA null, but may also express normal levels of a gene product that is nonfunctional by mutation.
Null alleles can have lethal effects depending on the importance of the mutated gene. For example, mice homozygous for a null allele for insulin die 48 to 72 hours after birth. Null alleles can also have beneficial effects, such as the elevated harvest index of semi-dwarf rice of the green revolution caused by null alleles in GA20ox-2.
Evidence
Polymerase chain reaction (PCR)
A microsatellite null allele is an allele at a microsatellite locus that does not amplify to detectable levels in a polymerase chain reaction test. Microsatellite regions are usually characterized by short, repeated sequences of nucleotides. In this analysis, a known parent was determined to be homozygous at a certain locus, but produced offspring that expressed a different "homozygous" genotype. In other words, a null allele can only be identified from the phenotypic standpoint if the individual is homozygous for the null allele. Researchers have been able to work around this problem by using detailed Electrophoresis, gel assays, and chromosomal manipulation.
- Allendorf et al. studied the enzyme activity of the same species of red pine seeds collected from two different tree stands in Minnesota. The two groups of trees were treated as one population because no deviations from expected genotype frequencies were observed, as would be expected if the populations were diverging from one another. Alleles that produced an enzyme lacking catalytic activity were denoted as null alleles. A total of 27 loci were tested in red pines and null alleles were found at 3 of those loci. Both experiments used the process of mutageneis, whereby the genetic content of the organism is changed, to produce individuals with different combinations of null mutations.
See also
- Pseudogene
- Muller's morphs
- Genetic deletion
- RecLOH
- Unique-event polymorphism