This article is from the Fall 1997 AFRMA Rat & Mouse Tales news-magazine.

By Ann-Zophi Pålsson and Eva Johansson, Sweden

ED. NOTE: This wonderful article was sent to us as part of the Tamråttans Vänner Yearbook. I found it extremely intriguing, and as permission was granted to reproduce it, we thought we would share it with everyone. The “Honung*” color equates to our Cinnamon Pearl. Many thanks to Ann-Zophi Pålsson and Eva Johansson for sending us the updated version of this article.

This color, an agouti variety, is to be a light golden orange brown with a shimmering impression (like pearl or golden pearls) and have black eyes.

This variety is one of the more complicated colors. The awl hairs** usually consist of three bands but it surely looks like they have four bands on this variety. The band closest to the skin or the undercolor (the under band) is creamish-grey and then slate blue a bit further out on the hair before the main band comes. We think that maybe this slate blue band isn’t a real band but belongs to the undercolor. The middle band (main band color) is strongly orange and the top band is creamish grey. The color at the tip of the hair (top band/ticking) is normally bred to be very small and almost not visible. The guard hairs (monotrich) are usually silver colored, but we have found guard hairs that consist of two bands, where the under band is creamy grey, and the upper band is silver. We have also found guard hairs with a light colored under band and with grey ticking. Honung shall not have 50% white hairs mixed with 50% colored hairs, since this is not a silvered variety.

In 1979, the first rats with this color appeared in England. The English name is Cinnamon Pearl and stems from the fact that the first rats born with this color sprang from a mating between Cinnamon (the color we call Kanel in Sweden) and Pearl (the color we call Pärl). The name is misleading though it is not a Cinnamon colored Pearl, more a curry colored Pearl.

The gene that causes this color is the latest known new mutation on the rat (at the moment). There is nothing like this known before 1978.

Many theories about what this gene is have been presented. The known geneticist Roy Robinson wrote in 1978 that the active gene is called Silver and has the symbol S (e.g. is dominant). It was thought that the gene had a connection with silvering (the same amount of white hairs mixed with the same amount of colored hairs), but it can be proved that silvering has nothing to do with this gene, so this theory was shown to be wrong in this instance.

It seems that there was a wish that the gene should be the same as the Pearl gene in mice, but mice don’t have the color Honung (Cinnamon Pearl) so that theory was false.

Since the color Pärl appeared before the color Honung, we call the gene Pearl (which is the English word for Pärl), not to confuse ourselves in the discussion, since it is certain that both colors are built on the same gene—the Pearl gene.

There is also another theory which we do not know the origin of, but which appears now and then. This theory states that the Pearl gene is dominant and has the symbol W. To make Pärl or Honung (Cinnamon Pearl), the Brown gene must also be present. The W gene is supposed to be lethal when homozygote (e.g. with deadly outcome). The WW rats die and only the Ww rats survive. The ww rats are normal colored. Litters with Pärl and Honung (Cinnamon Pearl) are therefore few in numbers.

This theory fails since the Pearl gene is not dominant but recessive (which is very easy to prove), also the symbol W is a known symbol of Dominant White (also called Dominant Spot) and should not be mixed up in this, especially since the W gene now seems to exist in Sweden.

That the Pearl gene is recessive can easily be proven in the following way: mate a Honung (Cinnamon Pearl) with a Viltbrun (Agouti) and you will get Viltbrun (Agouti). Hence proven recessive.

If we mate a Honung (Cinnamon Pearl) with another Honung, we get, to many a person’s surprise, Honung and Kanel (Cinnamon). Conclusion: The Pearl gene must be dominant since it can carry Kanel (Cinnamon) which then becomes recessive.

A lot of confusion, many theories, much complication, but the truth was a lot simpler than expected. Last year when we were thinking intensely about the Pearl gene, suddenly the genial but simple solution came up. How the Pearl gene would work.

We concluded that the Pearl gene was an allele to the Mink gene and thus is situated on the same locus, namely the M-locus. We knew before that the Pearl gene was dependent of the Mink gene. This was not so strange since the Pearl gene was a mutated Mink gene. We gave the Pearl gene the symbol mpe and it is recessive with the exception that it is dominant to m (the Mink gene). We have not yet been able to show if the gene is lethal or not.

Our theory is possible to prove by a complimentary test that shows if two different varieties are caused by the same gene or by alleles on the same locus. You just mate a Honung (Cinnamon Pearl) with a Kanel (Cinnamon) and you get Honung (Cinnamon Pearl), or mate a Pärl with a Safir (Mink/Lilac) and we get Pärl. If the Pearl gene had had a separate locus, we would have in these two matings instead gotten Viltbrun (Agouti) and Svart (Self Black in English). We also know that Pärl mutated from a line of Safir (Mink/Lilac) and the probability is that there is a connection between the Mink gene and the Pearl gene, which most people had noticed but did not seem to have understood. But that was not the end of the story. . .

During the year 1996, we got in contact with Roy Robinson, who we happily exchanged genetic theories with. We got the impression that he was very happy that we were so extremely interested in genetics. He sent us a copy of his paper.

Immediately we started to examine his theory and compare it to our own. The Honung (Cinnamon Pearl) genotype is mmPepe, Robinson says in his new theory. And, if you mate two Viltbrun (Agouti) carrying Safir (Mink/Lilac) and Pärl (MmPepe) with each other, you will get Viltbrun (Agouti), Kanel (Cinnamon), and Honung (Cinnamon Pearl). This shows that two different loci are involved if none of the involved genes are incomplete dominant.

Compare this to our theory: A Viltbrun (Agouti) colored rat can only carry Safir (Mink/Lilac) or Pärl in our theory. We can get all Viltbrun (Agouti), or we can get Viltbrun (Agouti) and Kanel (Cinnamon), or Viltbrun (Agouti) and Honung (Cinnamon Pearl). Our theory gives at least one less color in this mating.

Test matings done by Robinson show that you can in fact get all three colors in the same mating. This exact mating is not done in Sweden, but instead, the Svart X Svart (Self Black) mating is at least done once by an independent breeder. The result of this only mating was several rats of the color Svart (Black), a few Safir (Mink/Lilac), and a couple of the color Pärl.

Roy Robinson’s new Pearl theory seems to describe the genetics of these colors better than our theory did. The Pearl and Mink genes seem to be on separate loci, just as Roy Robinson states.

The color Honung is a very beautiful color and still quite popular in both Sweden and England. The Pearl gene does not only give the two colors Pärl and Honung, but also five to ten colors are possible to get. Roy Robinson says in his new Pärl theory, that the Pearl gene is epistatic to other color genes. This we know for sure is not true since we have seen Pärl in several colors like Choklad Pärl (Chocolate Pearl) and Selen Pärl (Blue Pearl), and we have also heard of a pink-eyed Honung.

* Honung means honey in English.

** The longest hairs in the rat’s coat (the ones that always are silver in the Silver Fawn) are called Monotrich and are usually the ones responsible for the ticking effect. (Ed. Note: Guard hairs.)

The hairs that make up most of the visible part of the fur are called Awls, these are the hairs that usually are banded with three bands (the agouti varieties especially). Awls are always a bit shorter than the Monotrich.

Besides these two kinds of hairs, the coat consists of at least one more kind of hair, the under hair which is shorter and usually not visible if you do not part the hairs.

Roy Robinson, Color Inheritance in Small Livestock, 1978.
Roy Robinson, Genetics of the Norway Rat, 1965.
Roy Robinson, “Mink and Pearl: New Color Mutants in the Norway Rat,” The Journal of Heredity 85(2): 142-143. 1994.
Nick Mays, The Proper Care of Fancy Rats, 1993.

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