“Pigtail,” a hereditary tail abnormality in the house mouse,Mus musculus

Summary

A skeletal abnormality, termed pigtail, in the house mouse is described which phenotypically resembles flexed tail. In a certain small proportion of litters, one or more young with spina bifida aperta occurred, especially in the more highly inbred litters. Genetically, pigtail is a recessive character, caused either by two complementary factors, or—more probably—by one main gene interacting with a number of modifiers which affect the penetration. Penetration is never 100%. In genetically pigtail litters, it varies round about 20%. It is not noticeably dependent on age of mother, degree of expression in the parents, degree of inbreeding or selection; but there exists a significant, though slight, negative correlation between litter-size and percentage of manifestation in homozygous litters, suggesting some intra-uterine, non-genetical influence on the development of the abnormality.