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Non-allelic homologous recombination (NAHR) is a form of homologous recombination that occurs between two lengths of DNA that have high sequence similarity, but are not alleles. [ 1 ] [ 2 ] [ 3 ] It usually occurs between sequences of DNA that have been previously duplicated through evolution, and therefore have low copy repeats (LCRs).
Gene conversion is the process by which one DNA sequence replaces a homologous sequence such that the sequences become identical after the conversion. [1] Gene conversion can be either allelic, meaning that one allele of the same gene replaces another allele, or ectopic, meaning that one paralogous DNA sequence converts another.
Allelic variation at a locus is measurable as the number of alleles (polymorphism) present, or the proportion of heterozygotes in the population. A null allele is a gene variant that lacks the gene's normal function because it either is not expressed, or the expressed protein is inactive.
These rules (patterns) are not without exceptions. Non-allelic mutants may occasionally fail to complement (this is known as "non-allelic non-complementation" or "unlinked non-complementation"). This is an uncommon occurrence that depends on the type of mutants being investigated. Two mutations, for example, could be synthetically dominant ...
Misalignment of LCRs during non-allelic homologous recombination (NAHR) [3] is an important mechanism underlying the chromosomal microdeletion disorders as well as their reciprocal duplication partners. [4] Many LCRs are concentrated in "hotspots", such as the 17p11-12 region, 27% of which is composed of LCR sequence.
Coalescent theory is a model of how alleles sampled from a population may have originated from a common ancestor.In the simplest case, coalescent theory assumes no recombination, no natural selection, and no gene flow or population structure, meaning that each variant is equally likely to have been passed from one generation to the next.
Ectopic recombination is an atypical form of recombination in which a crossing over takes place between two homologous DNA sequences located at non-allelic chromosomal positions. Such recombination often results in dramatic chromosomal rearrangement , which is generally harmful to the organism. [ 1 ]
Allelic exclusion is a process by which only one allele of a gene is expressed while the other allele is silenced. [1] This phenomenon is most notable for playing a role in the development of B lymphocytes , where allelic exclusion allows for each mature B lymphocyte to express only one type of immunoglobulin .