Twirler: A Mutant Affecting the Inner Ear of the House Mouse

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Abstract
The 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. Twirler heterozygotes show head-shaking in a horizontal plane, combined often with circling, and less often with vertical head-shaking. Postural reflexes are abnormal. For example, normal mice of about a week old, if held up by the tail, respond to this change of position by extending the back and stretching the forelimbs forward. Twirler mice, on the other hand, flex the back, tuck the head under, and draw the limbs back.