The coat is smooth and tight-fitting; colors are fawn, brindled, or white, with or without white markings. Boxers are brachycephalic (they have broad, short skulls), have a square muzzle, mandibular prognathism (an underbite), very strong jaws, and a powerful bite ideal for hanging on to large prey. Genetically, these dogs are either fawn or brindle, with excessive white markings overlying the base coat color. Like fair-skinned humans, white Boxers have a higher risk of sunburn and associated skin cancers than colored Boxers. The extreme piebald gene, which is responsible for white markings in Boxers, is linked to congenital sensorineural deafness in dogs. It is estimated that about 18% of white Boxers are deaf in one or both ears, though Boxer rescue organizations see about double that number. Median lifespan was 10.25 years. Responsible breeders use available tests to screen their breeding stock before breeding, and in some cases throughout the life of the dog, in an attempt to minimize the occurrence of these diseases in future generations.