There isn't different rat "breeds" just varieties and strains from the Norway rat that was domesticated.
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For actual standards check out American Fancy Rat and Mouse Association: http://www.afrma.org/fancyrm.htm
Most of the photos are mine. However I used a lot from online sources mentioned below:
http://ratvarieties.com/
eatdrinkplay.com
www.geocities.ws
www.nfrs.org
www.paperheartrats.com
goosemoose.com
ratsforsale.org
ratvarieties.com
