Light-colored dachshunds can sport amber, light brown, or green eyes; however, kennel club standards state that the darker the eye color, the better. Dapple and double dapple dachshunds can have multi-coloured “wall” eyes with fully blue, partially blue or patched irises due to the effect of the dapple gene on eye pigmentation expression. The breed is prone to spinal problems, especially intervertebral disk disease (IVDD), due in part to an extremely long spinal column and short rib cage. The risk of injury may be worsened by obesity, jumping, rough handling, or intense exercise, which place greater strain on the vertebrae. About 20–25% of dachshunds will develop IVDD.