Paralog

Paralog is a gene related to another gene in the same organism due to a gene duplication event. Paralogous genes, or paralogs, arise when a gene is copied within the genome. The copies can accumulate mutations and diverge functionally over time, potentially taking on new roles or partitioning ancestral functions. Paralogous genes are contrasted with orthologous genes, or orthologs, which are homologous genes in different species that diverged after a speciation event.

Paralogy contributes to gene family expansion and functional innovation. Duplication can occur through several mechanisms, including unequal crossing over during meiosis, replication errors, retrotransposition, and whole-genome or segmental duplications. After duplication, one copy may maintain the original function while the other is free to accumulate changes; in some cases both copies diverge or one becomes a pseudogene.

Paralogs can be categorized by their relationship to speciation into in-paralogs and out-paralogs: in-paralogs originate after a speciation event within a lineage and are typically more closely related to each other than to genes in other species; out-paralogs originated before the speciation event.

Examples include the globin gene family, where alpha and beta globin genes are paralogs within humans, and the Hox gene clusters, which contain paralogous genes arranged in conserved orders across vertebrates.

Identification relies on sequence similarity and phylogenetic analysis to reconstruct gene trees and distinguish paralogs from orthologs. Understanding paralogs clarifies how genomes diversify and how new functions arise through duplication.