垂直同源基因是指不同物種間的同源性基因，編碼的蛋白具相似或相同功能。
垂直同源基因 (orthologous genes) 和 平行同源基因 (paralogous genes)：
平行同源基因：一個物種內由同一祖先複製而來的多個基因，編碼的蛋白具有相似或相關功能。
外文注釋：
1.Waler Fitch, Systematic Zoology,1970,19:99-113:
Where the homology is the result of gene duplication so that both copies have descended side by side during the history of an organism,(for example, alpha and beta hemoglobin), the gene should be called paralogous(para = in parallel).
Where the homology is the result of speciation so that the history of the gene reflects history of the species(for example, alpha hemoglobin in man and mouse), the genes should be called orthologous.
2.Koonin, Trends in Genetics,1996,12:334-336:
By definition, orthologs are genes that are related by vertical descent from a common ancestor and encode proteins with the same function in different species. By contrast, paralogs are homologous genes that have evolved by duplication and code for proteins with similar, but not identical functions.
