Describing homologous genes that have arisen by duplication of an ancestral gene. The copies (paralogues) thus evolve side by side (‘in parallel’) in the genome of subsequent lineages, irrespective of speciation events, and hence display similarity of base sequence and, possibly, function. The class of homeotic genes known as Hox genes consists of 13 highly conserved paralogues, some of which have been identified in organisms as diverse as nematodes and mice. This is taken as evidence that they are the descendants of homeotic genes in an ancient common ancestor, possibly dating back to the Precambrian. Compare orthologous.