The Tc allele at the mouse T locus causes a tailless phenotype in mice, more severe than a deletion allele. It is not due to deletion of neighboring genes. When Tc is paired with a T-locus duplication, it is partially complemented, unlike a deletion allele which is fully complemented. This suggests that Tc acts as an antimorph, opposing the function of the wild-type allele at the T locus. The amount of Tc present correlates with the length of the body axis in mice.