The enamel is affected, but the pulp (which houses the nerve and blood supply) and dentin are normal. The reason why the teeth appear yellow is because dentin has a yellowish tint and is visible through the abnormally formed thin enamel.
One study (Rowley R, Hill FJ, Winter GB: An investigation of the association between anterior open-bite and amelogenesis imperfecta. Am J Orthod Dentofacial Orthop 81: 229-235, 1982) suggests that AI influences the growth pattern of the craniofacial skeleton. In other words, the shape of the skull and the occlusal bite (how the teeth come together) can manifest a certain pattern based on this dominant gene. The most common trait they found was that 24% of individuals with the AI gene have what dentists call "anterior open bite".
This X-linked inherited gene is transmitted to all males born to women with this gene and 50% of females born to a mother with this gene (Seow WK: Clinical diagnosis and management strategies of amelogenesis imperfecta variants. Pediatr Dent 15: 384-393, 1993).
The main problems associated with AI are aesthetics, sensitivity, and improper bite. The severity will vary between individuals. Aesthetic and sensitivity problems can be remedied with bonding or crowns in primary as well as permanent teeth. The bite can be corrected with orthodontic treatment.