Mice heterozygous for the semidominant curtailed allele of brachyury usually have only a short, boneless caudal filament in place of the tail, although a few have short tails. Some heterozygotes die between birth and weaning. Low-grade spina bifida and blood blisters just dorsal of the caudal filament, occasional hind-limb paralysis, or atresia ani have also been observed (Searle, 1965). Heterozygotes lack the odontoid process of the axis and intervertebral nuclei pulposi, except for traces in the lower thoracic and lumbar regions. There are wider than normal cervical vertebral centra and the atlas has an abnormal shape in the ventral arch. Missing and fused vertebrae, and fused and malformed ribs are also standard phenotypes. Ventral duplication of the neural tube or spinal cord also occur in heterozygotes.
Curtailed homozygotes die between embryonic day 9.5 and 10.5 with failed placental connections. They lack the posterior part of the body to a greater degree than do brachyury (T) homozygotes. Truncation of the primitive streak diminishes its insertion into the exocoelom, no allantoic core domain is then found in the allantois, and the anterior position of the allantoic-yolk sac junction results in placement within the embryonic compartment rather than extraembryonic. As a result, medial umbilical roots are foreshortened in homozygotes disrupting the organization of the vessel of confluence and umbilical artery (Rodriquez et al., 2017). Tc is dominant to Th but not to T. Tc/th7 compound heterozygotes have a tail that is shorter than normal, but present.
