Null allele

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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. A null allele cannot be distinguished from deletion of the entire locus solely from phenotypic observation.^[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. 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 due to 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.^[2] Null alleles can also have beneficial effects,^[3] such as the elevated harvest index of semi-dwarf rice of the green revolution caused by null alleles in GA20ox-2. ^[4]