Twirler: a mutant affecting the inner ear of the house mouse.

T H E mutant twirler, symbol Tw, of the house mouse, Mus musculus, was first recognized by the 'waltzing' behaviour which the heterozygotes show. It was later found to affect the morphology of the inner ear and, when homozygous, to cause death of new-born animals through harelip and cleft palate. This paper describes the behaviour and ear defects of heterozygotes and the appearance of homozygotes. It also describes the genetic tests carried out to determine the mode of inheritance.