Parents of a 7-year-old child addressed a hospital with complaints of the child having no permanent teeth in the front area of the mandible. Anamnesis states that the first deciduous teeth erupted at the age of 11 months. Objective clinical examination revealed the following: appearance is without changes; milk occlusion; there are physiological diastemas and tremas; edgeto- edge incisor contact. What preliminary diagnosis can be made according to Kalvelis classification?