Genome introgression from M. m. molossinus into classical inbred strains. European fancy mice originated from M. m. domesticus. In the late 18th century, a Japanese publication entitled “Chingan-sodategusa,” which means “How to breed fancy mice” (Tokuda 1935), reported small and spotted (piebald) mice reared by Japanese fanciers (lower right; courtesy of Kouwa-shyuppan, Tokyo, Japan). In the middle to late 19th century, British traders likely introduced Japanese waltzing mice carrying the “piebald” (Ednrbs) mutant allele to Europe. The ancestor of JF1, which was referred to as the Japanese waltzing mouse, was used for early studies of the Mendelism of its coat color and waltzing behavior. The mouse with the piebald phenotype (“a” in the top right photo) resembles the JF1 mouse (photograph courtesy of Carnegie Institution for Science, Washington DC, USA). Experimental crosses of the JF1 ancestor and European fancy mice conveyed the M. m. molossinus genome into the M. m. domesticus genetic background. Later, their descendants were transported to America (Keeler 1931; Morse 1981), where they were established as classical inbred strains.
