Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Genetic diseases are illnesses caused by anomalies in genes or chromosomes present from before birth. Most genetic diseases are rare and affect one person in every several thousands or millions. Some genetic disorders are passed down from the parents' genes, but others are always or almost always caused by new mutations or changes to the DNA. Learn more about genetic diseases, their types, causes and possible treatments in the following pages.