Fig. 1.

Four different types of homology relations. A family of five genes sampled from human (in blue) and mouse (in green) evolves through speciation and duplication events (left-hand tree). The relationships among genes are highlighted in the four trees on the right. Human genes H1 and H3 are inparalogs, arising from a duplication event after the most recent speciation event in the tree. Genes H2 and M2 are orthologs, as they are related through a speciation event. Gene pairs M1–M2 and M1–H2 are outparalogs because they arose from a duplication that predates the reference speciation event. Pair M1–M2 is within-species outparalogs, while M1–H2 is between-species outparalogs