As previously mentioned in this blog, there seems to be a recessive gene that causes Rattus rattus (Roof Rats) to have a white tail tip marking. Here are black (melanistic) and agouti roof rats with and without the marking.
Black Roof Rat with white tail tip marking
Black Roof Rat with a solid black tail
Agouti Roof Rat with a White Tail Tip marking
Agouti Roof Rat without any Tail Marking
The affected area is devoid of pigment, including the skin. I assume the gene prevents melanocytes from migrating to the tip of the tail if two copies are present. I’ve seen some indications that that individuals with just one copy of the gene have a very subtle marking at the very tip.
In many species, breeding for tamer animals also selects for more white markings, because the migration of melanocytes is correlated with neurological development in the fetus. So, it is not surprising that Roof Rats with white tail tip markings do seem to behave subtly different than those without.
For example, it seems that fewer males with the marking are successful at mating, possibly because they are slower or less aggressive than rats without the marking. But we would need to do a more careful study to be sure about this. Rattus rattus are generally not as successful at mating in captivity as R. Norvegicus, so the effect is not as obvious as it otherwise would be.
I find it curious that, so far, the pure white markings I have seen only affect the tail tips. Most R. Norvegicus markings affect a much larger area. Some Roof Rats have light colored bellies, but the affected areas are still somewhat pigmented, unlike (for example) Berkshire markings in Norway Rats, which are truly unpigmented.
Update: what the research actually says (2026)
Since I first wrote this, I had a deep dive done into the published science on white tail tips in rats. A few things turned out to be more interesting than I expected, and one thing I said above needs an honest correction.
Wild roof rats do get it, just rarely. I used to say I had never seen a wild roof rat with this marking. It turns out they are out there. In the Yambaru forest of northern Okinawa, about 8.7% of the wild rats carry white spotting (Kambe et al., 2012). Those Okinawan rats are technically the Asian house rat, Rattus tanezumi, a close cousin in the roof rat species complex. White tail tips and white patches have also turned up in true Rattus rattus on the Ogasawara (Bonin) Islands, in rats on Hainan and in Hong Kong, and as far back as 1967 in feral black rats from the Florida Keys and the Arabian desert. So this is a real, natural variation that shows up around the world, and it seems to have appeared independently in more than one rat lineage.
The gene still is not pinned down in roof rats. Researchers checked the usual pigment gene (Mc1r) in those Okinawa rats and ruled it out, both for the white spotting and for the black coats. (The wild black coloring there turned out to be a different gene, ASIP.) So the gene behind our white tail tip is genuinely still unknown. But here is the useful clue. In the domestic Norway rat, the ordinary pet “fancy rat,” there is a marking called “Irish” that is basically a white belly plus a white tail tip, and that one has been tracked down: it is the Kit gene, disrupted by a piece of an old virus sitting next to it that slows the pigment cells down. My best guess, and the researchers’ best guess, is that the roof rat tail tip is probably something in that same Kit pathway, but nobody has sequenced a roof rat to prove it.
It makes sense that the tail tip is where it shows up. The tip of the tail is the very last place the pigment cells reach while the babies are developing, so it is the first place to go white if anything slows those cells down even a little. The same white tail tip turns up in wild roof rats, in pet Norway rats, and in lab mice, all for the same reason. Different species, same weak spot.
The tameness connection: an honest correction. Above, I repeated the popular idea that breeding for tameness also brings more white markings, because the cells that make pigment are cousins of the ones that build the nervous system. That idea is real and famous (it comes from Belyaev’s tame foxes). But here is the honest catch. When researchers actually mapped the genes in tame versus aggressive rats (Albert et al., 2009), tameness landed on chromosome 1 and white spotting landed on chromosome 14. Different chromosomes, not linked. So at least in rats, a white tail tip is not proof that an animal is tamer. The two traits can travel separately.
In our own colony, the white tail tips trace back to one founder, Roofy, and they did become common as we bred for tameness. But that may simply be because they all came from her, not because tameness and the tail tip are genetically tied together. I honestly do not know yet. Because we know the whole pedigree of our line, it is one of the better places in the world to finally settle the question. My old hunch, that white-tip males seem a little less successful at mating, is exactly the kind of thing worth studying carefully rather than assuming.
If you work in genetics and have the tools to sequence this, please get in touch. It is a genuinely open question, and our line is ready-made to help answer it.
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