The Occurrence and Genetic Behavior of Duplicate Lower Incisors in the Mouse.

new trait in the mouse, involving the appearance of extra lower incisor A teeth, seems worthy of note because of its morphological interest and its unusual genetic behavior. The trait, which may be designated as di (duplicate incisors), is characterized by the appearance on one or both sides of a supernumerary and somewhat aberrant tooth located close to the mandibular symphysis and directly behind or slightly medial to the corresponding normal tooth. The accompanying figures show duplicate teeth as they appear in a living animal and in a cleaned lower jaw. A brief description of the trait, a few circumstances surrounding its origin, and some preliminary breeding records can be reported at this time. Normal lower incisors erupt at about the eleventh day of postnatal life and the accessories, with less regularity, a few days later, but their rudiments when present can be demonstrated in newborn and prenatal young. Each accessory has its own alveolus, which is so close to that of the normal tooth that the two may seem to emerge from the same socket. The duplicates are usually clavate at first and for a brief period may appear to be as large as, or even larger than the normals. Later, they tend to lose their bulbous tips and become smaller by comparison and more uniform in diameter. A few are closely appressed to the normal teeth with their tips correspondingly beveled, but most of them become deflected and are ultimately broken off or displaced from their sockets. When the latter happens, bone of the alveolar process soon fills in the deficiency making it difficult or impossible to say whether or not an extra tooth has ever been present. But not all duplicates are lost, some of them, or their broken roots, remaining indefinitely. Such persistent remnants often force the normal teeth sufficiently out of alignment to cause serious malocclusion, an eventuality that can be avoided if need be by early extraction of the duplicates. Thus far, aside from indirect effects of malocclusion, possibly somewhat reduced vigor, and a rather high incidence of mammary tumors, no indication has been found of other physical or physiological peculiarities that might be attributed to di genes.