Dogs differ greatly in their morphological characteristics including various tail phenotypes. Congenitally short-tailed dogs are present in many breeds; however the causative mutation located in the T-box transcription factor T gene (CI 89G) had only been described in the bobtailed Pembroke Welsh Corgis. We investigated here the presence of the T gene mutation in 23 other breeds (360 dogs, including 156 natural shorttailed) in which natural bobtailed dogs exist. In the 17 breeds in which the Cl 89G mutation was observed, there was a perfect correlation between this mutation and the short-tail phenotype. However 6 breeds did not carry the known substitution or any other mutations in the T gene coding regions. No dogs were found to be homozygous for the CI 89G mutation, suggesting that the homozygous condition is lethal. In order to study the effect of the Tgene mutation on litter size, we compared the number of puppies born from short-tailed parents to that born from long-tailed parents. In the Swedish Vallhund breed, we observed a 29% decrease in the litter size when both parents were short tailed. Given that the Tgene mutation is not present in all breeds of short-tailed dog, there must be yet other genetic factors affecting tail phenotypes to be discovered. [ABSTRACT FROM AUTHOR]