垂直同源基因 (orthologous gene) 和 平行同源基因 (paralogous gene)：
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.