Genes with a similar nucleotide sequence. Homologous genes are assumed to be derived from a common "ancestral gene". 2 types of homology of genes are distinguished:
- Paralogous genes (paraloga) are homologous genes in a species that arose by gene duplication and subsequent divergence of nucleotide sequences, such as the various hemoglobin genes (hemoglobins) in mammals.
- Orthologous genes (orthologs) are homologous genes in different species that are derived from a common gene in the last common ancestor of the groups under consideration.