WordNet

tending to undergo or resulting from mutation; "a mutant gene"
an animal that has undergone mutation
(biology) an organism that has characteristics resulting from chromosomal alteration (同)mutation, variation, sport
lacking any legal or binding force; "null and void" (同)void
the 13th letter of the Greek alphabet

PrepTutorEJDIC

突然変異種(堤)
(法律上)無効の,効力のない / ゼロの,無価値の / (数学で集合が)空(から)の
ニュー(ギリシア語アルファベットの第13字Ν,ν;英語のN,nに相当)

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2013/04/29 09:23:28」(JST)

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A null allele is a mutant copy of a gene that completely lacks that gene's normal function. This can be the result of the complete absence of the gene product (protein, RNA) at the molecular level, or the expression of a non-functional gene product. At the phenotypic level, a null allele is indistinguishable from a deletion of the entire locus.

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.

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English Journal

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Baltrunaite K1, Craig MP1, Palencia Desai S1, Chaturvedi P1, Pandey RN1, Hegde RS1, Sumanas S2.
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Progressive hereditary spastic paraplegia caused by a homozygous KY mutation.

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Japanese Journal

Geranylgeranyltransferase Cwg2-Rho4/Rho5 module is implicated in the Pmk1 MAP kinase-mediated cell wall integrity pathway in fission yeast

Doi Akira,Kita Ayako,Kanda Yuki,Uno Takaya,Asami Keita,Satoh Ryosuke,Nakano Kentaro,Sugiura Reiko
Genes to cells 20(4), 310-323, 2015-04
… Analysis of the vic2-1/cwg2-v2 mutant strain showed that the localization of Rho1, Rho4, Rho5 and Cdc42, both at the plasma and vacuolar membranes, was impaired in the vic2-1/cwg2-v2 mutant cells. … Consistently, the phosphorylation of Pmk1 MAPK on heat shock was decreased in the cwg2-v2 mutants, and rho4- and rho5-null cells. …
NAID 120005608185

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Hayashi Mari,Kodama Momoko,Nakamura Yasunori,Fujita Naoko
Journal of Applied Glycoscience 62(2), 81-86, 2015
… By contrast, a double mutant (ss1L/ss3a) generated by crossing the leaky ss1 mutant, whose SS activity is approximately ca. … 16% of the wild type, and the ss3a null mutant is fertile. … Although there is only a significant residual SS activity in the developing endosperm of ss1L/ss3a, the yield and growth of the double-mutant line is sufficient. …
NAID 130005070809

Thermal and Pasting Properties, Morphology of Starch Granules, and Crystallinity of Endosperm Starch in the Rice SSI and SSIIIa Double-Mutant

Hayashi Mari,Kodama Momoko,Nakamura Yasunori,Fujita Naoko
Journal of Applied Glycoscience advpub(0), 2015
… By contrast, a double mutant (ss1L/ss3a) generated by crossing the leaky ss1 mutant, whose SS activity is approximately ca. … 16% of the wild type, and the ss3a null mutant is fertile. … Although there is only a significant residual SS activity in the developing endosperm of ss1L/ss3a, the yield and growth of the double-mutant line is sufficient. …
NAID 130005060847