This article is from the Sep./Oct. 1988 AFRMA Rat & Mouse Tales news-magazine.
By Virginia Pochmann
After one solid year of trying to figure out the unusual mode of inheritance in our local strain of roan mice, I finally have some answers. I know several members of the club are now breeding these animals, and these findings may be of help to you. In the summer of 1987, I bought several roan mice from Jack Ball in San Jose, in whose stock this mutation seems to have originated. Several questions arose as soon as I began breeding them:
These questions were intriguing to an old exzoologist like me, so I devoted most of my cages to this strain of mice, in an effort to get some answers. I’ll take them in the order listed above, 1 through 4.
I puzzled and puzzled over this, and finally got the answer through the related question (no. 4) about why the dark marks begin and end at the midlines on the animals. While reading The Coat Colors of Mice, by Willys K. Silvers, I found a photograph on p. 154, showing a Lilac mouse with a similar pattern of black marks extending out from the midlines. This non-roan mouse’s marks were due to a gene called “p-unstable” (written as pun). This pun gene causes pink eyes and lilac coat color, (just like the usual (p) gene for pink eyes), but in this case is also unstable . . . meaning that during embryonic development, some of the pun/pun cells along the midline of the embryo (which would later give rise to the skin) could easily mutate back to P/pun which allowed the underlying color (Black) to take over the cell. When this occurred during early embryonic life, all the skin cells which subsequently grew from that one mutant cell would produce a black area on the Lilac mouse, which began or ended at the midline.
Now, I can’t prove it, but I am going to go out on a limb and say that I am convinced that this spontaneous reversion to underlying coat color (non-roan) is what is occurring here in our roan mice which have dark marks. The allele permitting this would then be “roan-unstable,” and could be written as (roun).
This explanation accounts for the fact that the dark-marked animals do not “breed true,” because it is purely a matter of chance which embryos will have patches of skin cells reverting to underlying skin color, and which will not. It also explains why many of the dark markings stop at the midlines, because the skin grows downward from the spine to the middle of the belly on each side of the embryo.
An animal showing patches of roan and non-roan in its coat is what is known in genetics as a “mosaic,” since its skin cells are of two distinct genotypes. The roan patches of skin actually have the roan gene in homozygous condition, and the dark-marked patches of skin do not. In the cells of the dark-marked patches, the genes have slipped back to heterozygous state (Ro/roun).
A Black Merle (dark marked Roan).
By continually selecting mosaic animals for use in matings, we may be concentrating unstable alleles . . . leading to a higher percentage of mosaic mice among the offspring . . . but obviously we will never get them to “breed 100% true,” since every dark mark must originate with a spontaneous reversion to non-roan (Ro) in a skin cell. There should always be some unmarked roans in the litters, in whom no “slippage” occurred during embryonic life.
These mosaic animals should be of great interest to scientists. I hope someone in the field of genetics will come forward and let me give him or her some of these fascinating animals to work with.