Homolog/ortholog

William R. Pearson wrp at dayhoff.med.Virginia.EDU
Tue May 10 09:57:57 EST 1994


In article <9405051451.AA20816 at thalamus.wustl.edu>,
Yamagata =?ISO-2022-JP?B?GyRCOzM3QRsoQg==?= <yamagatm at THALAMUS.WUSTL.EDU> wrote:
>How do you define a homolog, ortholog, or paralog?  Family, superfamily,
>subfamily, cognate, counterpart, related, homologous, --Do you use other
>words for describing relationships among homologous genes?  And, how would
>you use these words?  Is there good references for how we can accurately
>distinguish these words?  I know there is no simple answer for this.  But,
>I would like to know how molecular evolutionalists are using these words. 
>Thanks in advance.

Homolog (homologous) - shares a common ancestor

	ortholog - differences between two "orthologous" proteins reflect
		   a speciation event - mouse alpha-hemoglobin and human
		   alpha-hemoglobin differ because of the divergence
		   of the lines leading to mice and humands

	paralog - differences reflect gene duplication events, e.g.
		   human beta-hemoglobin and mouse alpha-hemoglobin.
		   An evolutionary tree based on chimpanzee alpha-globin,
		   human beta-globin, and mouse beta-globin would put
	  	   humans much closer to mice than chimps.

This definition is available in any textbook on molecular evolution.

The superfamily/family/subfamily hierarchy was defined by Dayhoff.
Typically members of a subfamily are more than 80% identical, family >
50%, and superfamily < 50%.  (Meth. Enz. 1990 183:37)


Bill Pearson



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