American Fancy Rat & Mouse Association

This article is from the WSSF 2015 AFRMA Rat & Mouse Tales news-magazine.

Colors & Coats

Dwarf Rat Info; Blue Genes Influence Size?; Small Rats

By Karen Robbins

Dwarf Rat Info

Veronica Collins, e-mail
Q My name is Veronica Collins, owner of Little Whiskers breedery. Recently here in Maine, a huge fuss has started over the History of the dwarf rat as a few breeders have talked about introducing them to our breeding lineups to make them available for a high-market clientele (not pet shops; as show rats and personal colonies).

Specific questions I would like to see answered so I can put at ease some people’s minds here in my community, are:

  1. What is a Dwarf rat?
  2. Where did they come from?
  3. How were they created?
  4. Is there any history to a rat and another rodent being crossed to produce a Dwarf?
  5. What are the risks in breeding Dwarf rats?
  6. Is there any inhumane history or circumstances at all to the Dwarf rat?

Thank you so much for your time! I hope to put this all to rest and help clear up some rumors of rats being bad pets!

A Here are some Internet pages that should answer your questions that have the history and scientific aspects of dwarf rats:

And the following pages are from breeders telling about their experiences with them that will answer your other questions:

Blue Genes Influence Size?

Sariah Lily Jones, Facebook
Q Are there any connections between either of the blue genes (in rats) with the rat’s size?

A In the article Influence of Certain Color Mutations on Body Size in Mice, Rats, and Rabbits by W.E. Castle, Genetics 1941 March; 26(2): 177–191, it talks about Blue (what we know as Russian Blue dd) being smaller than gray (agouti) or Cinnamon (brown gene + agouti A– bb) and the Cinnamons (brown agouti) were larger than either one due to the brown/chocolate gene.

In mice the Blue gene dd increases size where the leaden gene lnln (looks just like Blue) decreases size, and the brown gene (chocolate bb) increases size. Castle thought perhaps the rat blue mutation was not the equivalent of Blue in mice but possibly a similar phenotype such as leaden since the size was decreased.

I haven’t found this to always be the case though with my mice—some Chocolates are smaller and I’ve had small Blue mice and large leaden blue mice.

Small Rats

Jeremy Danes, Facebook
Q We have a group of fancy rats that only get 6 to 8 inches max body length. We have proven this out for 4 generations with only one throwback of regular size. The 8 inch body length one was a throwback, the rest aren’t over 6 inches in body length. When this genetic mini rat is bred with another genetic mini rat, we sometimes get a rat that only gets 4 inches max full grown. What would we have to do to get it proven as a new Standard? Besides photos and written documentation? The Dumbo in this size are adorable.

A Rats (and mice) can be selectively bred for many things, size being one of them. It’s hard to say if this is just from selection (choosing the smaller ones to breed) or an actual mutation. There are many standard rats that don’t get to the ideal 8–10 inches in body length because they have been bred from small stock over time. The genetic dwarf rats that came from a lab that a lot of pet breeders are breeding, come in different size and type variations themselves. One way to tell if this is a mutation, is to breed to a non-related rat to see if the small size comes through in the kids/grandkids.

To standardize new animals, they have to be different and unique from what is already recognized (and not have any physical problems or health conditions). A judge has to be able to look at the new animal and be able to see that it is different from anything standardized and not have to question if it is just a poor example of something else. Each club is different in how they accept new animals to their Standards. For AFRMA, you can read more on our Unstandardized page.

There are several dwarf rat genes—most cause physical problems, fertility problems, and a shortened lifespan. For more information on the history and scientific aspects and breeders’ experiences of Spontaneous Dwarf rats (dr; recessive gene) that pet rat breeders have, see the first answer.

While Spontaneous Dwarf rats have an increased lifespan, a small size due to a defect in the functional growth hormone cells (insufficient growth hormone) in the pituitary gland (making them dwarf), and less likely to get tumors, that doesn’t mean they won’t get tumors—most are pituitary and mammary tumors. The major non-cancer problems are incisor malocclusion (misalignment of teeth; often hereditary), heart disease, chronic nephropathy (kidney damage or disease; male), and cerebral hemorrhage (brain bleeding; female). Most cases of chronic nephropathy were mild.

Normal-size rats with selective breeding along with proper diet and environment, will have a reduced level of tumors and some will never get them. *

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Updated December 15, 2021