先看看paralogue
一生物體有兩組基因組 有著共同的來源 但卻有差異
像是人體中 alpha-hemoglobin beta-hemoglobin
這兩種蛋白質執行相同的功能
但卻屬於不同的基因組

orthologue
不同種間的兩組基因組
但是卻有相同功能
像是人和老鼠有部分的酵素是相同的
我門稱這兩種蛋白質的基因組
為orthologue

Homologous genes have shared a common evolutionary ancestor.
Note that
homology and similarity are not the same thing - genes which are
homologous may have high or low similarity, but the presence of
similarity doesn't necessarily make two sequences homologues (could be
convergent evolution).

Paralogous and orthologous genes are subdivisions of homologous genes.
All homologs are either orthologs or paralogs.

Orthologs arise because of speciation events.

Thus if a rat and a
human sequence have simply diverged since the last common ancestor of
rats and humans, then they are orthologs.

Paralogs arise because of gene duplication events.
Thus there may be an
alpha and beta form of a human protein which have arisen through gene
duplication, and then diverged.
These two genes are paralogs.
Also
note that if this duplication occurred before the last common human-rat
ancestor then alpha-human is a paralog to beta-rat, but an ortholog to
alpha-rat.