For some unknown reason the C. intermedius mice rarely breed in captivity, therefore tracing the gene responsible for color changes by any method that requires captive breeding, was nearly impossible. To try and identify the gene, the gene mutation responsible for color change in laboratory mice was examined. Lab mice typically have banded dorsal hairs, however it has been found that with mutations to the Agouti signaling proteins (12-14) and Melanocortin-1-receptor (Mc1r), the dorsal hairs will become unbanded. The laboratory mouse is a close relative of C. intermedius, thus by looking at the proteins responsible for color change in the common laboratory mouse the location of the color changing gene in C. intermedius can be estimated.
Doing this, for the Agouti proteins, 16 polymorphisms were found, but none of them seemed to have anything to do with coat color. However, when both alleles of the Mc1r gene was sequenced in 69 mice, 15 polymorphisms were found throughout them all, but 9 polymorphisms were only found in some of them. 4 of the 9 polymorphisms were seen only in the dark mice, so hoping that they would have a correspondence with coat color change, these were then examined. The mutations caused amino acid changes as follows:
(Note: the first column are the amino acids in the dark mice, and the second column is the amino acids in the light mice)
While there have been mutations in Mc1r that cause color change, these have never been found before. Furthermore, these mutations are only found in the Pinacate mice, but not the Amendaris mice, suggesting that the darker coat color mutation was independently evolved in each group of mice, despite the two groups being the same species.
The genes responsible for color change are receptors in a signaling pathway, and are more or less the master switch for light and dark color. These genes were then isolated and introduced into cultured human kidney cells to show that the dark allele codes for a different receptor than the light allele. The receptor on the cell works as depicted below:
When alpha-MSH binds to the cell receptor it causes the intracellular levels of camp to increase, which then stirs darker color. The amount of camp produced is dependant on how much alpha-MSH is attaches to the cell receptor, as seen in the following figure:
The amount of cAMP increases with increasing amounts of alpha-MSH to a certain point, because after awhile the amount of available cell receptors will supercede the amount of alpha-MSH, thus no more alpha-MSH can bind to the cell, and no more cAMP is produced. The amount of alpha-MSH that can bind to the cell before it reaches its threshold is much higher in the dark allele, and thus greater levels of cAMP can be made. The greater levels of cAMP is correlated to how dark the mouse's coat color is.
Genetics