MisterMiceGuy's Guide to the Science Fancy Mouse Breeding


One of my goals as a mouse breeder is to use mouse breeding to help others gain exposure to science and the natural world. I want to encourage and promote realistic animal breeding that’s based on science and I want it to be available to mature and responsible adults interested in these subjects. To accomplish this, I am writing this guide to becoming a fancy mouse breeder.


Mouse pups are cute but there are a lot of things that come with mouse breeding that are not cute. These include witnessing death, cannibalism and euthanasia. It’s not uncommon for a mouse breeder to find a dead mouse who has had its head eaten off by its cute cage mate. It’s also not uncommon to check in on your prized litter only to find that their mother has decided to eat the limbs off all her pups. It’s not for the faint of heart. In addition, there is also the reality of euthanasia. Although some might find it controversial euthanasia is frequently part of a breeding program that is carried out by the breeder themselves. This can be emotionally daunting, but some breeders feel it’s a necessary part of the hobby (Royer 1998). If you’re prepared to accept the realities of mouse breeding, then maybe you’re ready to explore mouse breeding as a hobby.



First thing you will need are appropriate cages for the mice and a place to keep those cages. If you plan on breeding multiple generations of mice you will need multiple cages to support a small colony of mice. Mister Mice recommends a minimum of 4 cages. One to house your breeder male, one to house a female with pups, and two to grow out your pups.

Some mouse breeders make their own cages out of large plastic bins by adding wire mesh to the sides and lid for ventilation.  Suitable plastic or glass cages can also be purchased but hamster-style cages are not recommended (FMBA 2019).  You can find instructions to make these online.  Remember that good cage ventilation is important as this helps reduce the buildup of toxic ammonia fumes (Smith, Stockwell, Schweitzer, Langley, & Smith 2004)

Recommended cage sizes for mice is based on the size of the mouse with larger mice needing larger accommodations. For Mice being housed in groups it is recommended that a 25g mouse have a minimum 15 sq. in. of floor space. The minimum height for a cage is for mice is 5in. The minimum floor space for a mother with pups is 51 sq. in. (National Research Council 2010). To figure out the size of your cage multiply the length (in inches) by the width (in inches) and that will give you total square inches.

You will also need a place to keep these cages. MisterMiceGuy suggests using a wire utility shelf or similar shelving system. The shelves can be adjusted so you have plenty of space to house your mice on top and then you can have space on the bottom to house your mouse supplies such as bedding or food.


Mice prefer beddings that contain large fibrous pieces (Blom, Van Tintelen, Van Vorstenbosch, Baumans & Beynen 1996). However not all large piece beddings are created equal. Untreated softwood beddings have been shown to affect the immune system and Cedar bedding has been associated with tumor development, altered liver enzyme functioning, and cytotoxicity in mice (Vlahakis 1977, National Research Council 2010 and Sabine, Horton, & Wicks 1973). Fancy Mice Breeders generally avoid cedar and other soft wood beddings (FMBA 2019). However, Kiln-dried pine maybe used as the kiln drying process reduces the aromatic hydrocarbons that are found in untreated pine. Special precautions should be taken regarding bedding used for hairless mice as their lack of eye lashes may contribute to conjunctivitis (Research Council 2010).

It’s worth noting that research has compared the ability of different beddings to control ammonia levels. This is relevant because ammonia is associated with disease in mice (Mexas, Brice, Caro, Hillanbrand, & Gaertner 2015). They found that the common paper-based bedding “Care fresh Ultra” performed significantly worse in controlling ammonia levels when compared to pine shavings, hardwood, or corn cob bedding (Smith, Stockwell, Schweitzer, Langley, & Smith 2004). Hardwood beddings, such as aspen shavings, are thought to be the best wood-based bedding for mice and are very good at controlling ammonia levels (FMBA 2019 and Smith, Stockwell, Schweitzer, Langley,& Smith 2004). It is common practice to replace bedding and sanitize cages weekly however some fancy mouse breeders recommend twice weekly cleanings (Mexas et al. 2015, AFRMA 2019)

Food and Water

Water bottles designed for small animals should be used to supply fresh water. Commercial “Lab block” style products designed for mice or rats can be used for food. Some breeders create their own food mixes (FMBA 2019). However, care should be taken as inappropriate macro nutrients, such as too little protein can have adverse effects on mice (Watkins et al. 2008).


When it comes down to it you have basically two options when it comes to getting animals to start with: pet store or private breeder. It’s common to find mice at pet stores but the quality of these mice can be lacking. It’s likely that these mice will have poor body types and poor temperament. You want to look for mice with broad heads and large ears in the coat color that you want. You will save a lot of time by starting off with a male that has the basic coat type and color that you want or that has exceptional type.

The preferred method is starting off with mice from a private breeder. You can locate private breeders on one of the fancy mouse breed club pages or by doing a google search. Again, its best to start with a male that closely matches the color and type that you are looking for. By purchasing from a breeder not only will you be starting off with higher quality animals but it’s likely that the breeder will know the genetic history of your animals. This can be useful if you have specific breeding goals.


Grasp the mouse by the tail near its base and place the mouse on a surface, such as your palm, that it can grab while maintaining your firm but gentle grip. Young mice should be picked up by scooping your hands around their entire body, or by picking up a group of baby mice along with some nesting material (Connor 2007).


Breeding mice is straight forward. Mice are old enough to breed between 4-7 weeks of age. Pregnancy in mice lasts 19-21 days. Once mice are born, they will wean off their mothers milk in about 21 days. Litter sizes vary from small litters of 4 pups to large litters of 12 pups. A female mouse’s breeding life span is from 6 to 12 months, and total life span can be anywhere from 1 to 3 years (Connor 2007).

Female pups should be separated from their male siblings and father between 3 and 4 weeks of age. If they are allowed to remain longer, they may become pregnant. Female mice that are bred before 6 weeks of age are still immature and may have difficulty giving birth or be more likely to eat her offspring. Additionally, females may start to loose fertility after about 6 months of age (Connor 2007).

When pairing mice you should make note of the pairing by documenting the pair, the date of the pairing, the estimated date of birth (Connor 2007). Once the pups are born you can go back and note the 3 week date and the 6 week date so that you know when to separate the pups and when they can be breed. Mister Mice suggests keeping a small note book for this purpose called a Breeder’s Log.

When caring for a pregnant mouse a higher fat diet may be provided for milk production. Handling of the female should be avoided for the first day or two after birth as this may cause the mother to eat her pups. Actually, any stressful events such as noises, aggressive handling, or overcrowding can lead to decreased fertility and increased chance the mother will eat her pups (Connor 2007).


When selecting mice to breed it’s important to have a goal in mind.  Ask yourself what do I hope to achieve with this pairing?  Generally speaking, there are two basic methods of selecting pairs.  One is that you pair animals with similar trains with a goal on improving that trait.  For example, you pair your largest eared mouse with your largest eared mouse with the goal of producing even larger eared mice.  The other method is pairing animals based on the traits that they don’t have.  For example, you have a large eared mouse with a narrow head, and you breed it to a mouse with a wide head but small ears with the goal of producing a mouse with a wide head and large ears or at least reducing what you feel is a fault in that mouse. 


There is a difference between breeding a pair of mice and breeding a line of mice. In order for your mice to “breed true” you need to develop a line of mice the consistently produces mice with the desired traits. If this is your goal, then you need think about setting up a breeding plan (Greenwood-Dudar 2018).

The first step is to identify the traits that you want to see in the mice you are producing (Greenwood-Dudar 2018). Traits can include any of the observable physical or behavioral characteristics (Barber 2005 and Genetic Learning Center 2016). If you plan on showing mice you might want to familiarize yourself with the varieties of mice and the characteristics associated with them (FMBA 2019, AFRMA 2019, and Greenwood-Dudar 2018). The second step is developing a feel for what those traits should look like in your mice. This includes being able to evaluate a given mouse and determine if it has the traits you are looking for (Greenwood-Dudar 2018).

As a new breeder you can increase your knowledge in these areas by joining a mouse breeders club, contacting local mouse breeders, or reaching out to other breeders on social media. If there are mouse shows in your area attending those might be helpful (Greenwood-Dudar 2018).


Growth: Mice develop rapidly between birth and 4 weeks and you will notice daily changes. On days 3-4 the ears will lift away from the head. On day 6-7 a coat of fine fuzzy fur starts to grow. On 9 day nipples are visible on females only. On days 10-11 their teeth break through their gums. On days 12-14 their eyes will open (Connor 2007).

Weaning: At about 3 weeks old mice will look a lot like tiny adults and will be ready to eat solid foods. If pups are separated too early from their mother, they may die so it’s important to know they are eating on their own before separation. One wat to test this to observe the pup’s behavior when you remove the lid to their cage. If the pups lay still, they are too young. However, if they run or jump around the cage then they are old enough to be separated (Connor 2007).


If you are a member of a mouse breeding club then take a peak of the club’s “standards” to see the traits that are desirable in show mice.  These are the traits you want to select for.  It might be helpful to google show mice to get an idea of what show mice look like.

There are two different stages in which mouse selection occurs.  Once when the mice are pups and another time when the mice are adults.  When the mice are adults it should be easier to tell if they have the traits you are looking for.  When you have a new litter it can be hard to tell if the mice will have the traits you are looking for.  Usually when selecting pups out of a litter you want to look at the width of the pups head and overall size of the pup.  Unfortunately, there are some traits (such as angora) you just won’t be able to select for until the mice have matured. 

Sex is also an important trait you want to pay attention to.  Male pups cannot be bred back to their father and they will fight with other males so breeders will often cull all their male pups. In fact, any pup that is small, sickly, or doesn’t have the traits you want needs to be culled from the breeding program.


Culling is simply removing an animal from your breeding population. It is sometimes divided into Hard Culling and Soft Culling. Hard Culling refers to euthanizing ill or unwanted animals. Soft Culling refers to removing of the animals by re-homing or by simply not using them in your breeding program any longer.

It would be nice if all the mice you produce were healthy and you could find homes for them. However, the reality is that mice produce far more offspring that you could possibly rehome. Additionally, in the wild most of these individuals would not survive. This high mortality rate in the wild is why mice produce so many offspring. In the captive breeding environment this makes culling a useful tool for the breeder.

There are two acceptable means of culling are Carbon Dioxide (CO2) for adult mice and Hypothermia for mice under 7 days old. The best way of using CO2 is by means of compressed gas cylinders as this allows you to regulate the flow of gas. Its preferable that mice be euthanized in their home cage but if not, a clean cage should be used (AVMA Guidelines 2013). Some breeders create their own CO2 gas using citric acid and baking soda. A 2-liter soda bottle system designed for aquarium CO2 injection might be used. Additionally, some breeders use CO2 cartridges and valves designed for brewing. (NEED CITATIONS)

Hypothermia is an acceptable means of euthanasia but only for mice under 7 days old (aka pinkies). This is because baby mice do not produce their own body heat. Death by hypothermia can be achieve by gradual cooling in a refrigerator. Mice should not be placed directly on cold or pre-cooled surfaces as this can cause tissue damage and potentially pain (AVMA Guidelines 2013).

Some mouse breeders cull unwanted pinkies in the belief that the pups that remain are healthier (Royer 1998). Research has indicated that pups from unculled litters have a reduced weight of 4% but that this difference disappears by 3 months of age (Paigen, Marion, Stearns, Harper and Svenson 2014). Additionally, some breeders believe that culling the males (which are larger) and leaving behind females will produce larger healthier females (Royer 2014).


There are many factors involved in being an ethical breeder (Royer 2015) Being an ethical breeder involves being concerned with animal welfare. Animal welfare is allowing animals to be healthy, comfortable, safe and free from fear and pain among other things (American Veternary Medical Association 2019). Being an Ethical Mouse Breeder involves breeding animals that are selected for traits that improve the health and appearance of animals and not breeding sickly individuals. It involves humane euthanasia of unwanted animals and using those animals in some resourceful way such as for food or education. Please see my article on Animal Welfare for more information on this topic.


Once you’re setup with your mouse breeding program you might consider joining a Fancy Mouse Breeders Club.  This will allow people to find you as a breeder, puts you in contact with other breeders, and allows you to find and participate in mouse showing competitions.  Two common clubs in the US are the Fancy Mouse Breeders’ Association and the American Fancy Rat & Mouse Association.  When you join one of this clubs you will select your official breeder name.

In addition to Joining a club you will want to setup a simple website, facebook, Instagram, or other social media account for your mice.  It can be simple with your contact information and location.  It should also describe the types of mice you are working on.


American Fancy Rat and Mouse Association (2019) Retrieved from: AFRMA.org

American Veterinarian Medical Association (2019). What is Animal Welfare? Retrieved from: https://www.avma.org/KB/Resources/Reference/AnimalWelfare/Pages/what-is-animal-welfare.aspx

AVMA Guidelines for the Euthanasia of Animals: 2013 Edition.  AVMA Guidelines. Retrieved from https://www.avma.org/KB/Policies/Documents/euthanasia.pdf

Blom, H.J.M., Van Tintelen, G., Van Vorstenbosch, C.J.A.H.V., Baumans, V. and Beynen, A.C., 1996. Preferences of mice and rats for types of bedding material. Laboratory animals30(3), pp.234-244.

Connor, A.B. (2007) Aurora’s Guide to Mouse Colony Management at MIT. Retrieved from https://ki.mit.edu/files/ki/cfile/sbc/escell/mouseManagement.pdf

Hermenegildo, C., Marcaida, G., Montoliu, C., Grisolía, S., Miñana, M.D. and Felipo, V., 1996. NMDA receptor antagonists prevent acute ammonia toxicity in mice. Neurochemical research21(10), pp.1237-1244.

Mexas, A.M., Brice, A.K., Caro, A.C., Hillanbrand, T.S. and Gaertner, D.J. (2015) Nasal histopathology and intracage ammonia levels in female groups and breeding mice housed in static isolation cages. Journal of the American Association for Laboratory Animal Science54(5), pp.478-486.

National Research Council (2010) Guide for the care and use of laboratory animals. National Academies Press.

Paigen, B., Marion, M.A., Stearns, T.M., Harper, J.M. and Svenson, K.L., 2014. The effect of culling on health and physiology of mouse litters. Laboratory animals48(3), pp.207-215.

Royer, N. 1998. Culling: The question that has plagued the fancy for many years. American Fancy Rat and Mouse Association. Retrieved from: http://www.afrma.org/culling.htm

Royer, N. 2015. A Responsible Breeder’s Code of Ethics. American Fancy Rat and Mouse Association. Retrieved from: http://www.afrma.org/breedethics.htm

Royer, N (2014) Culling: The question that has plagued the fancy for many years. American Fancy Rat and Mouse Association. Retrieved from http://www.afrma.org/culling.htm

Sabine, J.R., Horton, B.J. and Wicks, M.B., 1973. Spontaneous tumors in C3H-A vy and C3H-A vy fB mice: high incidence in the United States and low incidence in Australia. Journal of the National Cancer Institute50(5), pp.1237-1242.

Smith, E., Stockwell, J.D., Schweitzer, I., Langley, S.H. and Smith, A.L., 2004. Evaluation of cage micro-environment of mice housed on various types of bedding materials. Journal of the American Association for Laboratory Animal Science43(4), pp.12-17.

Vlahakis, G., 1977. Brief Communication: Possible Carcinogenic Effects of Cedar Shavings in Bedding of C3H-Avy fB Mice. Journal of the National Cancer Institute58(1), pp.149-150.

Watkins, A., Wilkins, A., Cunningham, C., Perry, V., Seet, M., Osmond, C., Eckert, J., Torrens, C., Cagampang, F., Cleal, J. and Gray, W. 2008. Low protein diet fed exclusively during mouse oocyte maturation leads to behavioural and cardiovascular abnormalities in offspring. The Journal of physiology586(8), pp.2231-2244.

Genetic Learning Center (2016). Inherited Human Traits: A Quick Reference. University of Utah. Retrieved from: https://health.utah.gov/genomics/familyhistory/documents/Family%20Reunion/reference%20guide.pdf

Barber, N. (2015) What Behaviors Do We Inherit via Genes? Psychology Today. Retrieved from: https://www.psychologytoday.com/us/blog/the-human-beast/201509/what-behaviors-do-we-inherit-genes

Greenwood-Dudar, A. (2018) Establishing A Bloodline: Selection Of Brood Stock And Breeding Styles. American Dog Breeders Association. Retrieved from: https://adbadog.com/establishing-bloodline-selection-brood-stock-breeding-styles/

The Roan Gene

A gene that MisterMiceGuy is working with is the Roan gene. In order to learn more about this gene an inquiry was made to the Fancy Mouse Breeders’ Association private group that revealed there may not be much research that has been conducted into the Roan gene and that much of what is written is just conjecture (Sampson 2020). Additional comments indicated that laboratories may call the gene roan unstable (roun) and that there are two versions of the recessive Roan gene (Laigaie 2020, Wyss 2019).

The Roan mice currently circulating in the Fancy Mouse Hobby were initially found in a feeder colony owned by Jack Ball of San Jose, CA and was developed by him in the 1980s (Robbins 2014, Emerson 2020b). In 2009 descendants of Ball’s Roan mice were sent by Mike Choido of New York to Dr. Roland Fischer in Germany. It’s reported that all Roan mice in Europe come from this line of mice (Robbins 2014). When it was first discovered the gene was referred to as “Jack Ball Roan” (Emerson 2020a).

Example 1. Recessive Roan mouse exhibiting the typical white ticking “roaning” throughout the coat (MisterMiceGuy 2020)

According to Ball (1986) most genetic textbooks list the roan gene as lethal. The exact text books are not listed and MisterMiceGuy was unable to locate them. Lethal in this context seems to mean that homozygotes will fail to be born or will die shortly after birth (Ball 1986). The Roan gene that MisterMiceGuy is recessive and does not appear to be a lethal gene. This may indicate that there are multiple roan-like genes in circulation.

A literature search on Google Scholar and Medline for Roan Mice, Unstable Roan Mice, and Merle mice only returns one article by De Sepulveda, Guenet, and Panthier (1995) that discusses a “roan effect” gene (Wrio). This gene appears to be dominant and occured spontaneously in an inbred strain maintained at the Oswaldo Cruz Foundation in Rio de Janeiro, Brazil. Interestingly this gene is both dominant and lethal in its homozygous state which is similar to the gene in textbooks described by Ball (1986). Unfortunately, this gene is not the gene that MisterMiceGuy is working with because it is dominant. Although it is not the same, it does have a similar appearance and it is possible that this gene functions similarly to the recessive gene that is in MisterMiceGuy’s possession.

W-locus “Roan Effect” mouse from the Oswaldo Cruz Foundation exhibiting phenotypic reversion around the left ear (Photo Credit: De Sepulveda, Guenet, and Panthier 1995).

Confusingly, in addition to reports of there being two recessive roan genes (discussed later) there are also reports that both genes have variable expression. In some instances a roan mouse will have a coat that has a base color with white furs even distributed covering its entire body (see example 1). In other instances the mouse will have a different phenotype consisting of roan fur and solid patches (see example 2). This phenotypic variant is called Merle (Pochmann 1988). The solid patches are called “phenotypic reversions” because the coat is reverting back to its non-roan state (De Sepulveda, Guenet, and Panthier 1995). Animals with this phenotype are also known as mosaics (Pochmann 1988) but the mosaic term does not seem to be in common use in the fancy mouse community. Both versions of the recessive gene have this variable phenotype.

These solid patches are not symmetrical (De Sepulveda, Guenet, and Panthier 1995) and generally start and end at the midline (Pochmann 1988). They also seem randomly positioned (De Sepulveda, Guenet, and Panthier 1995) and not traditionally heritable (Pochmann 1988). Both Pachmann (1988) and De Sepulveda, Guenet, and Panthier (1995) compared the expression of the Roan gene to that of the pink-eyed unstable (pun) gene.

Example 2. Recessive Roan exhibiting asymmetrical areas of pigmentation also known as phenotypic reversions (MisterMiceGuy 2020).

In the case of unstable genes such as pun, or potentially the recessive roan genes, the DNA is damaged in during the migration of cells during embryonic development. The DNA is only altered in certain cells through processes like hemizygous deletion or mitotic recombination (De Sepulveda, Guenet, and Panthier 1995). It was hypothesised by Pochmann (1988) that in the case of recessive Roan one of the alleles is lost randomly as the DNA is replicated and cells migrate from the midline across the embryo. The loss occurs at random leaving certain cells in a heterozygous state (a non-roan state) allowing pigment to be produced but only in select areas. This would explain why the pattern of solid patches in roan mice are not heritable and random.

Additional searches reveal that there are a variety of genes that produce white hairs in the coat such as Flecked (Fk), Freckled (Fkl), dominant roan (Rn), Varitint-Waddler (Va), Silver (si), and Misty (m) (Pochmann 1988 & Sviderskaya, Novak, Swank, and Bennett 1998). Based on photos and descriptions none of these are the same as the recessive Roan gene that is circulating in the American mouse breeding hobby today.

There have been reported to be two apparently distinct recessive Roan genes with similar phenotypic expression reported by Fancy Mouse Breeders. One has been identified as type 1 (early type) and the other is type 2 (late type) (Ball 1986, Wyss 2019). However, both variants seem to come from the single spontaneous mutation discovered by Ball in the 1980s (Emerson 2020b). If the gene developed as a spontaneous mutation in Ball’s colony it would seem highly unlikely that he would have had two instances of spontaneous mutation resulting in two genes with similar phenotype. MisterMiceGuy suspects that all of the phenotypic variants of the recessive roan gene (early, late, Roan, and Merle) come from the variable expression of a single gene.

The early type is described has having the roan pattern in place when the coat first comes in. It is distinguishable from late type because the white hairs will already be mixed in to certain patches of fur when the first coat develops and the animal will not change significantly as it ages (Ball 1988). This early type Roan development matches the hypothesis that Roan development has a similar mechanism to Pink-eye unstable pun as explained by Pochmann (1988). MisterMiceGuy has noticed that although his mice have been identified as the early type (Wyss 2019) it seems that the mice are born with mostly regular pigmentation and that the Roan pattern develops quickly after the coat grows in. This suggests that the Roan allele is not lost randomly during cell migration in the embryo. Perhaps what is randomly altered during embryonic cell migration is some sort of genetic factor that affects both A. if a cell will loose pigment and B. when a cell will loose pigment.

The late type as you might expect has poorly defined areas of roan and solid coloring when the coat first comes in. By the time the mouse is four or five weeks the coat rapidly fades to a lighter roan pattern revealing distinct solid patches in certain areas (Ball 1988). Being that in late type Roan the cells have pigment early on and then lose it suggests that the DNA is not lost during during cell replication during development and migration. It would seem that these cells maintain their homozygous state through embryonic development and then at some point later the DNA or melanocytes are lost. This supports MisterMiceGuy’s hypothesis that both early and late type recessive roan are the same gene and that there are additional modifiers affecting if a cell will lose pigmentation and when the pigmentation will be lost.

Dominant Spotting W-locus gene showing random and asymmetrical pattern development similar to Recessive Roan (Photo Credit: Geissler, McFarland, and Russel 1981)

Additionally, MisterMiceGuy suspects that the Recessive Roan gene may be located at the W-locus on chromosome 5 and not a hypothetical “Ro” locus as is seemingly suggested on the Fancy Mouse Breeder groups and websites (Bernstein et al. 1990, Laigaie 2020, The Finnish Mouse Club 2020).

This is hypothesis is supported by the idea that W-locus genes are known to affect cell differentiation and coat color changes. Of particular interest is that the W-locus is known to have genes that produce pleiotropic developmental defects (Bernstein et al. 1990). Pleiotropic means that a single gene is responsible for a variety of phenotypes (Sheil 2020) which seems to be what we are observing in the Recessive Roan gene (Ball 1986, Pochmann 1988).

Similar effects can be seen on other W-locus genes whose phenotypes exhibit random and asymmetrical patterns such as the “Roan Effect” mice from Rio or Dominant white spotting W-locus mice (De Sepulveda, Guenet, & Panthier 1995, Geissler,McFarland, Russel, 1981).

Despite having complex and unknown genetic origins the Roan gene produces variable and stunning mice that are valuable contributions to the mouse breeding hobby. Additionally the roan gene can combine with many of the other coat color dilution genes such as black, chocolate, red, or even chinchilla to produce surprisingly beautiful mice (Ball 1986).

Updated March 25, 2020


Pochmann, V. (1988) Explanation of Roan Mouse Inheritance Factors. American Fancy Rat and Mouse Association. Retrieved from: https://www.afrma.org/roanmiceinh.htm?fbclid=IwAR36Ida8GpNQQGn0lHcafavg1MDkhT29BwbzZJlMsUjpPutYCWDFCJkGMBI

Ball, J. (1986) Breeding Roan Mice. American Fancy Rat and Mouse Association. Retrieved from: https://www.afrma.org/roanmice.htm

Geissler, E., McFarland, E., Russel, E. (1981) Analysis of pleiotropism at the dominant white spotting (W) locus of the house mouse: a description of ten new W alleles. Genetics, 97, 337-361.

De Sepulveda, P., Guenet, J. L., & Panthier, J. J. (1995). Phenotypic reversions at the W/Kit locus mediated by mitotic recombination in mice. Molecular and Cellular Biology15(11), 5898–5905.

Laigaie, M. (2020, March 19) Merle is ro^un (roan unstable). retrieved from: https://www.facebook.com/groups/mousebreeders/?multi_permalinks=1350454325157523&notif_id=1584629546420063&notif_t=feedback_reaction_generic [Facebook update]

Sampson, K. (2020, March 19) Most is just theories. retrieved from: https://www.facebook.com/groups/mousebreeders/?multi_permalinks=1350454325157523&notif_id=1584629546420063&notif_t=feedback_reaction_generic [Facebook update]

Wyss, J. (2019, December 11) There are two versions. Early and late. You have early. Personal Facebook Message.

Sviderskaya, E. V., Novak, E. K., Swank, R. T., & Bennett, D. C. (1998). The murine misty mutation: phenotypic effects on melanocytes, platelets and brown fat. Genetics148(1), 381–390.

Robbin, K. (2014) Mouse Genetic Questions. American Fancy Rat and Mouse Association. retrieved from https://www.afrma.org/roanmice.htm

The Finnish Mouse Club (2020) Varieties. Retrieved from: http://www.hiiret.fi/eng/breeding/?pg=4&sub=11&ala=8

Bernstein, A., Chabot, B., Dubreuil, P., Reith, A., Nocka, K., Majumder, S., Ray, P., Besmer, P. (1990). The mouse W/c-kit locus. Ciba Foundation symposium, 148, 158-66.

Shiel, W. (2020) Medical Definition of Pleiotropic. MedicineNet. Retrieved from: https://www.medicinenet.com/script/main/art.asp?articlekey=4942

Emerson, M (2020a) We called it Jack ball roan. Facebook post retrieved from: https://www.facebook.com/groups/mousebreeders/1355472471322375/?comment_id=1355493717986917&reply_comment_id=1355498774653078&notif_id=1585151197268579&notif_t=group_comment_mention

Emerson, M (2020b) “Both are from him and descended from the original female he pulled out of his feeder colony.” Personal Facebook message.

The Rex Gene

MisterMiceGuy is focusing on breeding mice who have curly/wavy hair. The gene responsible for this trait is called Rex (Re) and is located on the chromosome 11 (Crew & Auerbach 1939, Sundberg and King Jr 1996). The Rex gene is also commonly known Astrex (The Finnish Mouse Club 2020).

A short haired young adult male exhibiting slightly wavy hair due to being heterozygous for the Rex gene (Photo Credit: MisterMiceGuy 2019).

Rex mice were originally described by Crew and Auerbach (1939) when they received some mice from a Mr. Tuck who worked at the Rayleigh Rat and Mouse Farms in Essex, England. The gene was named after Rex Rabbits that had a similar coat texture (Crew & Auerbach 1939).

Today, in the fancy mouse hobby, there are inconsistencies regarding the casual naming of mouse genes and phenotypes (Robbin 2018, The Finnish Mouse Club 2020). Regardless of its casual name, the Rex gene should not be confused with a variety of other similar but distinct genes such as frizzy (Fr), Caracul (Ca), Wellhaarig (we), Waved-1 (wa-1), or Waved-2 (wa-2) (Spacek et al. 2010, Carter 1951). Mice with curly fur of unknown genetic origin are sometimes described as “Rexoid” (Carter 1951).

(A) Newborn mouse with typical closed eyelid (Toonen, Liang, and Sidjanin 2012). This would be the typical eye lid of a mouse with the Rex gene. B) Newborn mouse with open eyelid due to wave-2 gene (Photo Credit: Toonen, Liang, and Sidjanin 2012).

Rex and Caracal are described as being very similar or identical in phenotype but originating from different genes (Crew & Auerbach 1939). Rex and Waved-2 occur on the same chromosome and are loosely linked (Carter 1951). There are some interactions we between Rex and the waved-1 and waved-2 varieties (Carter 1951). A notable difference between Rex and waved-2 is the presence of open eyelids at birth (Toonen, Liang, and Sidjanin 2012).

Non-Rex pups can be identified early due to their straight whiskers (Photo Credit: MisterMiceGuy 2019).

Although once thought to a be a complete dominant autosomal gene (Crew and Auerbach 1939) it has been since discovered that the Rex gene has incomplete dominance (Carter 1951). Heterozygous mice (ie mice with one copy of the Rex gene) will have smoother coats, looser waves, and more widely spread whiskers. Homozygous mice (with two copies of the Rex gene) have rough coats, whiskers that heavily curve inwards towards the mouse and have been described as “walrus” like (Carter 1951).

Baby heterozygote displaying whiskers that turn away from the face at the tip (Photo Credit: MisterMiceGuy 2019).

The difference between the phenotype of heterozygous and homozygous Rex mice has been thoroughly described by Carter (1951). On Day 7 Homozygous mice can be identified by heavily forward curled “walrus-like” whiskers and guard hairs on the neck and rear that curl forward. Heterozygous mice have whiskers and guard hairs that appear longer and straighter and are curled outwards just below the tip of the hair shaft (Carter 1951). As of March 2020 MisterMiceGuy does not have any homozygous mice to use to illustrate the “walrus-like” whiskers. Pictures to come as soon as homozygotes are produced.

Juvenile male heterozygote displaying the loose ripple wave pattern (Photo Credit: MisterMiceGuy 2019).

On Day 11 through 20 homozygous mice have a ripple wave pattern that appears earlier, is tighter, and has a rougher appearance when compared to heterozygotes. Carter (1951) states that homozygotes might have a crest that develops along the midline on the dorsal and ventral surfaces however MisterMiceGuy has noted that this crest also appears in heterozygotes. This may be due to hobbyist having increased the quality of the Rex curls by selective breeding. Regardless the midline crest tends to disappear with age (Carter 1951).

A young adult male heterozygous for the Rex gene. The curled whiskers and coat have straightened considerably with age (Photo Credit: MistermiceGuy 2019).

By 3 weeks the coat tends to lose much of its curl and ripple effect but homozygous individuals will retain more of the curl and will have a rougher appearance than heterozygous individuals (Carter 1951)

The Rex gene is a fun and easy gene to work with due to its semi-dominant nature. This is especially true for new breeders as breeding a homozygous rex to a non-rex will produce all rex pups and breeding a heterozygous rex to a non-rex will produce half rex pups (Carter 1951). For dramatic effect, Rex can also be combined with long haired genes to produce Texels (Robbins 2018).

Young adult male that is homozygous for long hair and heterozygous for the Rex gene (Photo Credit: MisterMiceGuy 2019).


Carter, T. (1951) Wavy-coated mice: Phenotypic interactions and linkage tests between rex and (a) waved-1, (b) waved-2. Journal of Genetics, 50, 268–276. https://doi.org/10.1007/BF02996223

Crew, F.A.E., Auerbach, C. (1939) Rex: A dominant autosomal monogenic coat texture character in the mouse. Journal of Genetics, 38341. https://doi.org/10.1007/BF02982178

Robbins, K (2018) Texel Mice (a.k.a. Long Haired Frizzie). American Fancy Rat and Mouse association, retrieved from https://www.afrma.org/c-c_texelmice.htm

Spacek, D., Perez, A., Ferranti, K., Wu, L., Moy, D., King, T. (2010) The mouse frizzy (fr) and rat ‘hairless’ (frCR) mutations are natural variants of protease serine S1 family member 8 (Prss8). Experimental Dermatology, 19, 527-532.

Sundberg, J (1994) Handbook of Mouse Mutations with Skin and Hair Abnormalities: Animal Models and biomedical tools. CRC Press, pp. 407.

Sundberg, J., King Jr., L (1996) Mouse Mutations as Animal Models and Biomedical Tools for Dermatological Research. Journal of Investigative Dermatology, (106)2, 368-376.

The Finnish Mouse Club (2020). Genetics. Retrieved from: http://www.hiiret.fi/eng/breeding/?pg=5&sub=2&fbclid=IwAR2Hc-ZyIRHDt0o3_zY5pCiRMkkpEaW2EvKTIm2PwGpe418Qu63WGmOFdDU

Toonen, J., Liang, L. & Sidjanin, D.J. (2012) Waved with open eyelids 2 (woe2) is a novel spontaneous mouse mutation in the protein phosphatase 1, regulatory (inhibitor) subunit 13 like (Ppp1r13l)gene. BMC Genet 13, 76. https://doi.org/10.1186/1471-2156-13-76