paralogues

Paralogues are genes related by duplication within a genome. They originate from a common ancestral gene that has been copied, after which each copy may accumulate mutations and diverge in function. Paralogues are distinct from orthologues, which are genes in different species that descended from a single gene in the last common ancestor. The term paralogue (paralog) derives from Greek para, “beside,” and logos, “relation.”

Gene duplication can occur through unequal crossing over, replication errors, retrotransposition, or whole-genome duplications. Duplication creates redundancy; over time, one copy can maintain the original function while the other is freed to explore new functions or expression patterns. This can lead to subfunctionalization (partitioning of ancestral functions), neofunctionalization (acquisition of a new function), or nonfunctionalization (loss of function, becoming a pseudogene).

In-paralogues and out-paralogues: in-paralogues arise by duplication after a speciation event, within the same lineage; out-paralogs arise prior to the speciation and may be present in multiple species. Gene families often consist of many paralogous genes that may form clusters on chromosomes or across species due to shared duplication events.

Examples include the globin gene family, where paralogous genes such as alpha- and beta-globin evolved after duplication, and the Hox gene clusters, which contain paralogous members from successive duplications in vertebrate evolution.

Methods for identifying paralogues rely on sequence similarity and phylogenetic analysis, often supplemented by synteny information to distinguish paralogy from orthology and by models that account for gene duplication in comparative genomics.