American Fancy Rat & Mouse Association

This article is from the Sept/Oct 1996 AFRMA Rat & Mouse Tales news-magazine.

Rat Genetics, part 2

By Nichole Royer

Last issue we covered the first half of the rat color spectrum. This month I am covering the second half. Please remember, I do not claim to be a geneticist. Some of the information contained in this article is theoretical. This article is based on the writings of other authors (in particular those of Ann Storey and Nick Mays), combined with the experiences of many breeders. Some of this information is, at best, an educated guess. We may find evidence to change the current thoughts on a particular color at any time.

M–Mink Locus

This locus causes black to be diluted to a gray-brown color, which is distinctly different from Chocolate.

M–Full color (no dilution)
m–Mink (black diluted to gray-brown)

mm causes eumelanin to be gray-brown, creating Minks and Lilacs in aa animals. The difference between the two appears to be due to polygenes, with Lilac being a lighter version of Mink. The color is extremely variable and can range from very light gray through dark brown, often with purple or blue highlights. This gene on Agouti causes Cinnamon.

*Me–Merle Locus

This particular locus is so new that we know very little about it; however, I am including it here because it does exist (this is another one that isn’t in the genetics books). It causes irregular blotches of dark color to appear on a lighter colored background (very similar to merle in dogs).

Me–Merle (blotched)
me–Non Merle

Merle appears to be dominant, so MeMe and Meme animals show blotching, meme animals do not. So far, the only colors we have seen Merle in are Lilac, Silver Lilac, and Pearl. On Lilac, the spots are almost the same color as the rest of the coat, and are very hard to see. On Silver Lilac, the spots are distinct when they are babies, but fade at their first molt then, every time they molt, the spots get darker. On Pearl, the spots are extremely clear, and it is hoped that they will remain so throughout the animal’s life. So far it has not been possible to combine Merle with any other colors. The one attempt produced Blue, Siamese, Blue Point Siamese, and one Lilac baby, of which only the Lilac was Merle. It will be very interesting to see if we can get Merle in other colors, and I will write more on this as information becomes available.

Merle Rat

P–Pink-Eyed Locus

This locus has a strong diluting effect on eye color and black/brown pigment, creating pink eyes and a pale yellow color. It only slightly affects yellow/red pigment.

P-Black eyed (no dilution)
p-Pink eyes (black/brown diluted)

In Black, Chocolate, Lilac, and Mink animals pp dilutes the color to various shades of Champagne (pink-eyed Blacks are show Champagnes). pp dilutes the black band in Agouti to champagne creating Silver Fawn and Amber (which is the same as Silver Fawn, just selected for a lighter color). Pink-eyed Blue (ggpp) is a white with a cool blue cast—a color we call Silver*.

Pe–Pearl Locus

Pearl is very recently described in the literature, though the fancy has known about it for quite some time. It only affects Mink and Lilac, in which it causes most of each hair to be white with just the tip being colored.

Pe–Pearl (hairs white with colored tip)
pe–normal color (hairs have no white)

In Mink and Lilac animals Pe is responsible for the colors Pearl and Silver Mink/Lilac. Pearl is dominant, though PePe animals die before or shortly after birth, so every Pearl x Pearl litter will contain some Lilac or Mink animals. The amount of white on each hair can range from quite a lot to relatively little. The lighter range we call Pearl, while animals whose hairs have considerably less white we call Silver Mink/Lilac. The Lilac/Mink color can also vary from very light to quite dark. In Cinnamon animals, Pe causes the the bottom layer of the blue-gray band at the base of the hairs to lighten to a pale cream, while only lightening the top layer slightly. Because it does not affect the red pigment in the coat, the top band is unaffected and remains orange, The dark guard hairs are affected in the same way as the base of the hair, and turn nearly white. This gives us the very pretty color Cinnamon Pearl.

R–Red Eyed Locus

This locus is very similar in affect to P, but not quite as intense. It dilutes eyes to ruby, lightens black/brown, and slightly affects red.

R–Black eyed (no dilution)
r–Ruby eyed (black/brown diluted)

rr causes Black animals to be diluted to Beige and Agouti to Fawn. Rr often causes black group animals to have slightly lighter coats and ruby eyes as youngsters, and it can make agouti group animals brighter.


There is no separate locus that controls the rest of our silvers. Silver Black and Silver Chocolate are caused by the same set of polygenes that cause a small amount of silvering in all colors of rats. Our show quality silvers have just been selected for this feature.

The major exception to this rule appears to come from the same gene which causes Dalmatian spotting. All Dalmatian rats which come in colors that can be silvered, are very silvered. When bred to non-Dalmatian rats, Dalmatians often produce babies which are heavily silvered Berkshire type blacks who have white collars partially around their necks. This gene needs more study before we can even begin to make a guess at what all is involved.

(To be continued . . . part 3)

  • “Genetics of Fancy Rats” by Ann Storey, Pro-Rat-A, The National Fancy Rat Society Journal, November/December 1995, Number 90, pp.7–11 (This is the most current and up-to-date information available on the genetics of fancy rats. She includes much more information than I have been able to present and I highly recommend anyone interested in this subject get a copy. There will be a more detailed version of this article appearing in the next NFRS Handbook which will be out in 1997. I am anxiously awaiting its publication.)
  • “Genetics” by Ann Storey, National Fancy Rat Society Handbook, a Pro-Rat-A publication 1991, pp 30–35. (Also published in various issues of Pro-Rat-A and AFRMA Rat and Mouse Tales, this article was the old standby for genetics information. It contains most of the basics and is well worth reading.)
  • The Proper Care of Fancy Rats by Nick Mays, T.F.H. publications, Inc., 1993 pp. 226-53. (Contains an excellent section on how genetics work and is a must read for anyone just being introduced to the world of genetics.)
  • “Genetics for Mouse Breeders” series by Bonnie Walters, published in various issues of AFRMA Rat and Mouse Tales. (If you ignore the fact that it is talking about mice, this is the best information on how genetics work that is available. This series of articles contain the most complete and detailed explanations of the workings of genetics, how and why breedings produce what they do, and what to expect when crossing one thing to another. If you ignore the information on color genetics, which are different for mice than they are for rats, it’s a very valuable resource.)
  • The Inheritance of Coat Color in Dogs by Clarence C. Little, Sc. D., Howell Book House 1971. *

Go to Rat Genetics, part 1
Go to Rat Genetics, part 3
Go to Rat Genetics - The WHAT IF chart

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Updated December 19, 2015