Each of us is the result of genes inherited from our parents and the influence of our environment on the functioning of the genes and body components encoded in our genes. Sometimes an error occurs during replication of genes as part of the reproductive process. Most of the errors are analogous to typos in printed text -- a single wrong character appears in an instruction in a gene. The defective gene results in the production of a faulty component for the body. Because the defect in the gene is accurately copied in subsequent reproductive cycles, the defect in the body component is inheritable. The defect is called a hereditary, or genetic, disease. Because so-called genetic diseases are not caused by an infectious agent, genetic diseases are not transmitted through contact.
Hereditary macular degeneration, better known as juvenile macular degneration (JMD) is suspected when macular degeneration occurs in more than one relatively young member of a family, and in more than one generation of that family, where the genetic relationships of the affected members is consistent with the rules of inheritance of genes. Hereditary forms of macular degeneration, which affect children and young adults, are much less common than age-related macular degeneration.