Homology/similarity/identity: proper usage.

D. Pat Beckfield beckfdp at pallas.network.com
Thu Jan 31 10:57:13 EST 1991

In article <3824 at gazette.bcm.tmc.edu> steffen at mbir.bcm.tmc.edu (David Steffen) writes:
>  I am again struggling with the proper use of the words "homology",
>"similarity", and "identity" in comparing sequences.  Specifically, we
>have cloned and sequenced (a bit of) the rat homologue of the _lck_
>gene.  The sequence of the mouse and human _lck_ genes is known.  How
>do we know what we have is the rat homologue?  Because when we compare our
>sequence to the published sequence, most of the nucleotides can be made
>to match up with minimal futzing.  So how do I say that?  
		  [discussion of rationale deleted]
>David Steffen
>Department of Cell Biology, Baylor College of Medicine, Houston TX 77030
>Telephone = (713) 798-6655, FAX = (713) 790-0545
>Internet = steffen at mbir.bcm.tmc.edu

As you're discussing semantics, it seems appropriate that I (a writer and
BS in zoology) respond.

The first question is does the rat sequence you're working with execute
the same function as the similar sequences for mice and humans?  If you
don't know, is there a way to test it?  Until you know this, what you
have is a homomorphic string -- similar structure, but not necessarily
having the same function.

If they do carry out the same functions, but you're still concerned by
evolutionary relations, you can call them "analogous" -- having the
same function, but not necessarily the same origin.

I hope this helps.

D. Patrick Beckfield                          pat.beckfield at network.com
7600 Boone Ave N                              (612) 424-4888
Network Systems Corporation

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