In the early 1950s, a wild type female from the Namru strain was crossed with a bald hrba/hrba male and one of the pups produced was a male with a bent tail. Pedigree tests revealed the underlying mutation to be semidominant and carried on the X chromosome. The predominant phenotypic trait of kinked, shortened tails results from mis-formed, smaller, and absent tail vertebrae. Heterozygous females have varied expressivity spanning a broad phenotypic range including mice with no apparent phenotype and mice with multiply kinked and shortened tails. Hemizygous males have the highest expressivity; the tail is shortened in some cases to half the normal length and in the most severe cases the kinks in the tail will cause the tail to bend sharply. In males the tail kinks are more common in the distal than the proximal half of the tail. Heterozygous females are fertile, but hemizygous males and homozyous females have decreased viability and fertility. The number of affected offspring is lower than expected. This is believed to result from in utero death as evidenced by an increase in resorption. The kinked and shortened tails likely result from neural tube defects. More than 10% of BNT/Le embryos have exencephaly. Orofacial schisis, eye defects, abnormal transition from a lordotic to a kyphotic curvature during neurulation, and omphalocele have also been found in these embryos. Mutant mice are smaller in overall body size and have an increased incidence of an interfrontal bone. As with ZIC3 mutations in humans, situs ambiguus was found in more than 50% of Bn/Y males and 38% of carrier females. (Garber ED, 1952, Science; Garber ED, 1952, PNAS; Gruneberg H., 1955; Johnson DR, 1976; Gebbia et al., 1997; Klootwijk et al., 2000.)
Non-disjunction was found to occur in the BNT/Le-a/a Bn/+ strain at a frequency of at least 4.4% and is thought to be a characteristic of this inbred background rather than due to the bent tail mutation. When female heterozygotes were crossed to Mus castaneus no bent tail phenotype was found in more than 40 F1 female offspring examined, which the authors point to as an indication of the importance of modifier loci in the expression of this phenotype. (Carrel et al., 2000)
