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Physical loss of nucleotides without replacement from genetic material.
Deletions are a type of mutation. Whether from genes or, more generally, from chromosomes, deletions create genetic though but not physical gaps in DNA (or, for RNA viruses, in RNA).
These gaps are noticeable particularly upon comparison with related base sequences, either from ancestral organisms (e.g., parental strains) or from different but similar organisms (e.g., related species).
Genetic material that has been deleted is difficult to recover via mutation. Mutational reversions of deleted alleles thus tends to not happen. Pseudoreversions, a.k.a., compensatory mutations, may still be possible, however, that is, mutations in different genetic material from that which has been deleted. Alternatively, genetic material that has been deleted my be replaced via recombination such as in association with horizontal gene transfer.
Given sufficient divergence of lineages prior to comparison, note that it can be difficult to distinguish between deletion of once shared genetic material in one species versus insertion of new genetic material in the other. Also difficult to see are shared deletions, that is, when both organisms possess the same or similar deletion.
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