The enormous complexity of the brain's structure and function allows many possibilities for genetic errors to occur. The same is true for the eye, an extension of the brain.

Some of these errors are known as structural defects. One such defect, holoprosencephaly, results in a single brain ventricle where a pair usually occurs, and malformation of facial features, including cyclopia and the lack of a nose. Although such cases are often sporadic, they sometimes follow a Mendelian pattern. Hydrocephalus, which results from too much cerebrospinal fluid for the size of the brain, enlarges the ventricles, causing intracranial pressure. It can be X-linked or multifactorial in origin. In some 2-3/10,000 live births, the neural tube of the embryo fails to close completely. This can cause varying sensory and motor defects, depending on which part of the spinal cord and nerves are affected. The condition may...