Abstract
Hairpin-tail (T(hp)) is a new allele of brachyury on chromosome 17 (linkage group IX) of the mouse. The morphological effects in the heterozygote are on the notochord and the tail. The homozygous abnormal is unknown. T(hp) is unique in that the phenotype of the heterozygote seems to depend on the source of the T(hp) gene. If it is inherited via the egg the phenotype is extreme and death follows in utero. Embryos derived from a T(hp) sperm and a+egg are less abnormal and viable. This is not a simple maternal effect, as T(hp)xT(hp) matings produce two distinct types of heterozygous embryos.