Orthologs and Paralogs are two types of homologous sequences. Orthology describes genes in different species that derive from a common ancestor. Orthologous genes may or may not have the same function.Paralogy describes homologous genes within a single species that diverged by gene duplication.
NCBI的Glossary中对paralog的定义是： A paralog is one of a set of homologous genes that have diverged from each other as a consequence of gene duplication. For example, the mouse a-globin andb-globin genes are paralogs. The relationship between mouse a-globin and chick b-globin is also considered paralogous (see the figure).
因此不能说：来自不同物种的就不是paralog。这一点就与前面的定义不一致了，前面说：“Paralogy describes homologous genes within a single species that diverged by gene duplication.”
NCBI的Glossary中对ortholog的定义是： Orthology describes genes in different species that derive from a single ancestral gene in the last common ancestor of the respective species.
TRENDS in Genetics Vol.18 No.12 December 2002 的一篇文章：《Orthology, paralogy and proposed classification for paralog subtypes》对两者的定义做了进一步的探究。作者指出：
Paralogs are defined as genes that derive from a single gene that was duplicated within a genome.The latter definition does not specify that paralogs can only be found in a single organism, and hence genes in different organisms that arose from gene duplication in an ancestral genome are also paralogs according to the definition。
As we know ,there are two kinds of homolog, ortholog and paralog. Genes in two species that have directly evolved from a single gene in the last common ancestor are called orthologs. A set of homologous genes that have diverged from each other as a consequence of genetic duplication are called paralogs. Sometime those paralogs which arose from a duplication after the speciation event are called in-paralogs.