Yorkshire Terriers are very playful and energetic dogs. Many people who have a Yorkie as a pet have two, because they often have separation anxiety when left alone and 'they don’t enjoy being alone.' . Adult Yorkshire Terriers whose coat colours deviate from the standard, or that have woolly or extra fine coats, are still Yorkshire Terriers. In addition, care may be more difficult for 'woolly' or 'cottony' textured coats, or coats that are overly fine. For example, a mature Yorkie may have a silver-blue with light brown, while another might have a black and creamy colour. Certain genetic disorders can be found in Yorkshire Terriers, including distichiasis, hydrocephalus, hypoplasia of dens, Legg–Calvé–Perthes syndrome, luxating patella, portosystemic shunt, retinal dysplasia, tracheal collapse, and bladder stones.