The Mystery of Manx: The Tailless Gene in Mice

by May Snyder of Mus Rodentry. Article last updated 5/4/2004.

Manx, also known as "tailless", is a gene in mice which causes a shortened tail. When bred towards shorter and shorter tails, a mouse with absolutely no tail can be created.

Manx in mice (at least in my own experience) is a dominant gene with additional modifier(s) which control exactly how long the tail should be. You need the main manx gene to see any tail shortening. It works line an on-off switch; without the main gene the tail is always full length.

Manx is both one of the easiest things to breed while also one of the hardest. Because manx is dominant, one can get immediately see results in the very first generation, even when one of the parent is not a manx. In just two or three generations of careful breeding you can go from slightly kinked tails to totally tailless, even if you had to breed to a non-manx to begin your lines.

Manx is hard to work with and in some ways dangerous. The gene doesn't always just affect the tail. In extreme cases it shortens the spine, affects how the hips connect to the spine, and changes spinal curvature. This means the babies may be born unable to use their back feel well or in some cases totally unable to walk. Totally tailless females may have birthing problems, depending on how her pelvis and spine were affected. For this reason, beginners to breeding should avoid working with manx. The best way to become prepared in breeding manx is to have a good, experienced manx rat or mouse breeder show you in person examples of manxs, how to inspect their spine, and how to decide when it is/isn't safe to breed a manx girl.

Starting Out

Your first challenge will be finding a good source of manx mice. Beware -- sometimes a normal mouse whose tail was removed by accident looks like a manx. If you're buying from a pet shop or broker, they may honestly not know if the mouse is manx or not.

Expect to travel a bit to get your first manx mice. Also expect to pay a bit more than usual for them, since manx mice are often priced high because of their unusual nature. The most I have paid for Manx was about $15 for a pair.

Although most people breeding mice are honest, the extra money manx mice bring also increases the chance they're not being bred properly.

Warning signs: claims the manx mice are "purebred"; this is impossible since mice do NOT come in breeds. Warning: Claims the totally-normal looking mice are "carriers"; all manx mice I've ever seen in the fancy get it from a dominant gene. A dominant gene, by definition, means the animal cannot carry the gene without showing it... so these mice simply could not have the gene without you seeing something.

Sometimes you'll see a mouse without a tail in a pet shop. The vast majority of these are ones whose tails have been removed at some point in their life. The hardest "fakes" to tell from real manx are those whose tails were removed while still pinkies. Without examining a few real manx in person, it's hard to really get a feel for what to look for. I've included a few drawing to give you some guidelines:

	Probably NOT manx A long, slender tail with a perfect tapering at the end, no shortening of length, no kink at end of tail.
	Probably NOT manx: Tip of tail is blunt, no sign of change in the actual vertebrae. Possibly a scab or scar from the injury. (see photo of yellow mouse above)
	Probably a manx Tail is shortened but is tapered, not rounded. No sign of injury or scarring at tip Look for crook at end of tail where last few vertebrae are a little off-center.
	Almost certainly a manx "Pigtail" is a tiny tab of skin at base of tail with no bones in it. No signs of scars or scabs. Possibly affected hips (look for wider stance or a difference in ability to run).
	Might be a manx but... If the mouse has absolutely no tail at all, there should be some signs in the hips and/or spine he or she really is a manx. Observe his/her gait. A manx with zero tail often has changes in spine and hips and will hop rather than run. He/she may also stand with feet pointed outwards.

Breeding

Your first step in breeding manx is to find good, health breeding stock. Be aware some manx lines are heavily inbred. This may or may not be a problem, since inbreeding isn't inherently bad. Inbreeding can bring out latent problems, however, so the inbreeding should be done with extreme care. Inbreeding isn't a "yes" or "no" question, but rather a measure of degrees: how related is one mouse to his mate? It's reasonable to ask the breeder how inbred the mice are. A great breeder should be able to provide you with accurate pedigree charts going back generations.

If you have to choose between getting one male or one female to start with, you will be better off with a male. In totally tailless females, there is always the slight chance in birthing or fertility problems. Ideally it's best to get several manx mice, preferably from slightly unrelated lines. This allows you the freedom to stop one line if a male is producing less than perfect babies.

You can mate a manx to a manx, but you don't need to in order to produce more. A manx bred to a regular mouse will give interesting results. You will see babies with 1/2 tails, some with 1/3 tails, some with 9/10th of a tail, and some with something in between. ALL of these babies are technically manx because they all have the manx gene. Most of them, however, will be nowhere close to the ideal manx standard.

In order to get better results, select your shortest tailed girl and breed her to the shortest tailed boy you have. Again, do not use females with 100% tailless-ness, any spinal shortening, trouble walking, etc. Even if she is able to give birth, you may find you've gone too far with the gene, and the result could be mice with cannot walk. Over time, with careful selective breeding, you'll find you're getting a higher and higher percentage of no-tailed mice in a litter.

In your breeding you will notice a variety of tail lengths. Some will be almost totally tailless with little curly pigtails (without bones). Some will have really short tails, much like a hamster. Others will have half or more of their tail, but they will have a kinked last few vertebrae at the end of the tail and/or a slightly shorter length of tail for their size. These mice should make healthy pets, but I do not recommend breeding from those with the longest tails. You will end up just making more kink-tailed mice without getting any closer to your goal.

Be aware if a mouse is genetically tailless, it should be so at birth. Tails which have tips that fall off later in life are not manx mice. It's exciting to examine your pinkies right after birth and be able to tell right away who is manx.