Nonhomolog

Nonhomolog is a rarely used term in genetics that is typically encountered as a noun in phrases or as an informal shorthand for sequences or structures that are not homologous to a reference sequence. A homolog is a DNA sequence or chromosome that shares ancestry and substantial sequence similarity with another. A nonhomolog, therefore, lacks such similarity to the comparative sequence and is used to describe sequences that do not have a detectable homolog in a specified genome or set of genomes, or structures that do not share a common ancestry with a given counterpart.

In practice, the concept appears in comparisons of genomes or genes, where researchers distinguish between homologous regions (conserved across species) and nonhomologous regions (no detectable similarity). It also appears in discussions of mechanisms like non-homologous end joining, a DNA repair pathway that joins broken ends without requiring sequence homology, and nonhomologous recombination, which refers to recombination events that occur between non-identical sequences and can lead to chromosomal rearrangements. The term can be used to describe nonhomologous chromosomes, i.e., chromosomes that are not a homologous pair, and thus do not carry the same gene set in a diploid organism.

Because nonhomolog is not standard on its own, most literature will use nonhomologous or specify the context (nonhomologous chromosome, nonhomologous end joining). Related concepts include homologs, orthologs, and paralogs.