The ulnaless mutation (Ul) causes severe abnormalities in all four limbs, but no abnormalities in the axial or cranial skeleton in either heterozygotes or homozygotes. Heterozygotes have an extreme reduction in the size of the radius and ulna with deformed fibulas, which are shortened and detached from the tibia, and tibias, which are generally bowed, and tibia and fibula are generally rotated approximately 90 degrees. Male heterozygotes fail to mate on this predominantly B6;C3H genetic background, but they appear to have normal gametogenesis and Peichel et al successfully used sperm from heterozygous males in in vitro fertilization. Fewer than the 50% expected heterozygotes are observed in the offspring of heterozygote x wildtype crosses on this background and this is due to prenatal and neonatal death. Females are more likely to die perinatally than are males. Homozygotes, assessed on a genetic background that included MOLF/Ei and FVB/N, have a more severe phenotype than heterozygotes, affecting the same skeletal components. The radius is shorter, there is no ossification center of the ulna, and there is a more severe loss of the fibula.
