The particolour coat is white with black-blue and tan. The white is caused by the recessive piebald gene. It is very rare to get a particolour, and if one is found, it tends to be very expensive. Some Yorkshire Terriers are solid golden, they only produce pheomelanin, others are liver or chocolate, a brown colour; they produce brown eumelanin instead of black eumelanin. The standard prescribes clearly defined fur-colors, and non-standard colours may indicate crossbreeding with other breeds or in rare cases even health problems. The AKC registration form for Yorkshire Terriers allows for four choices: blue and tan, blue and gold, black and tan, black and gold. Colour alone will not affect whether or not a dog is a good companion and pet. Distichiae, eyelashes arising from an abnormal spot (usually the duct of the meibomian gland at the edge of the eyelid), are often found in Yorkies. Distichiae can irritate the eye and cause tearing, squinting, inflammation, corneal abrasions or corneal ulcers, and scarring.