JIGR/Dn is maintained with jittery (Atcayji) and grizzled (Mfsd12gr) in repulsion.
Atcayji homozygotes can first be identified at approximately 12 days of age by a leaning, zig-zag gait when they attempt to run and by the difficulty they have righting themselves when placed on their backs. Disease progression is rapid such that within two more days mutants are found to crouch on their heels in a squatting position and can not run without falling. Within three to four days of the first symptoms, tetany is seen during exertion or excitement. This is initially most pronounced in the forelegs, which the mice beat up and down during these two- to three-second spasms. Failure to gain weight is seen by the third week of life and it is unclear whether this results from an inability to take in food. The severity and frequency of tetany increases, and during the fourth week weight loss and increased weakness precede death. The mean age of death is approximately 31 days. Tetrahydrobiopterin levels in the brain and GTP cyclohydrolase activity in the liver are lower in Atcayji homozygotes than normal. At 20 days of age, serum calcium is within normal ranges. Parabiotic twins made with an affected and an unaffected sibling do not transfer a mutant phenotype to the normal partner indicating that a diffusible factor does not cause the mutant phenotype. However, tetany and weight loss are inhibited in the mutant partner which also has increased longevity, yet still develops failed muscle coordination and a crouching posture. (DeOme, 1945; Snell, 1950; Duch et al., 1986.)
Mfsd12gr is a recessive mutation that occasionally causes tail kinks and consistently causes dilution of the yellow pigment but not the black pigment of the hair. The coat color has been described as similar to chinchilla (Tyrc-ch/Tyrc-ch) but with the black pigment remaining undiluted. On the agouti JIGR/Dn background the Mfsd12gr/Mfsd12gr coat color is grayish agouti. On a non-agouti background the hair in the ears and around the genitalia is white. The Mfsd12gr mutation causes 40-50% mortality prior to phenotypic classification and this affects males more than females. This mortality is both postnatal and prenatal from approximately 10 days onward. Pregnant dams expected to carry some homozygotes have been found to carry some dead embryos some of which had craneofacial abnormalities including shortened snout and swollen optic and occipital regions. At birth homozygotes weigh an average of one quarter less than their wildtype siblings. Although they increase in weight as suckling pups, as adults they still weigh 5-25% less than their wild type siblings. (Falconer, 1950; Bloom and Falconer, 1966.)
