Genetical Studies on the Skeleton of the Mouse 1 XXII . The Development of Danforth ' s Short - tail

T H E semi-dominant gene for Danforth's short-tail in the mouse (symbol Sd; linkage group V) was first described by Dunn, Gluecksohn-Schoenheimer, & Bryson (1940). The most conspicuous abnormality of Sdl + heterozygotes is a shortening of the tail the extent of which varies with the genetic background (Dunn, 1942; Fisher & Holt, 1944; Dunn & Gluecksohn-Schoenheimer, 1945). Reduction or absence of kidneys is common on some genetic backgrounds, but rare or absent on others (Gluecksohn-Schoenheimer, 1943). Reduction or absence of the dens epistrophei (odontoid process of the axis) with formation of an anomalous articulation between atlas and epistropheus (axis) was later described by Theiler (1951 a, b\ 1952; 1954) and by Griineberg (1953). The reduction of the dens epistrophei is part and parcel of a general reduction of the vertebral bodies which is most marked in the cervical region, but which can be traced throughout the whole length of the axial skeleton. SdlSd homozygotes (Gluecksohn-Schoenheimer, 1943) are either completely tailless or have a tail filament only; in the absence of rectum and anus there is a persistent cloaca; the bladder is reduced or absent, as is the urethra and the genital papilla; the kidneys are usually completely lacking, and the SdlSd homozygotes almost invariably die within 24 hours after birth. It has been claimed that on certain genetic backgrounds, the SdlSd homozygotes may have enough functioning kidney tissue to permit survival for a longer period (Fisher & Holt, 1944), but the evidence for this assertion has been criticized by Dunn & Gluecksohn-Schoenheimer (1945).