There is a certain confusion between two similar medical terms that are even used interchangeably: genetic disease and hereditary disease. Despite the similarities between these terms, not every genetic disease is also a hereditary disease.
What are genes?
Genes are protein production “recipes”, or codes, that exist in all the body’s cells. They determine various traits, such as eye color, vulnerability to certain diseases, and even our sensitivity to UV radiation.
Our genes are stored inside the cell nucleus. From there, they instruct the cell which substances it must produce. For instance, eye color is determined by a sequence of four genes. However, some traits are determined by only one gene. Genes are inherited from our parents: half are inherited from our mother and the other half from our father.
What are genetic diseases?
If certain abnormalities occur in the genes, this may be reflected in a genetic disease. In many cases, one mutated gene is enough for a genetic disorder to develop. However, these mutated genes are not always inherited.
Certain genes can be inherited but undergo changes during one’s life. Skin cancer, for instance, can develop due to exposure to radiation that causes genome abnormalities. The source of this disease are gene mutations, but the disease was not inherited from one’s parents.
In addition, genetic diseases can be caused by mutations in the parents’ egg or sperm. In this case, the disease is not hereditary since hereditary diseases are passed down from generation to generation. If the defected gene is only found in the mother or father’s germ cells (reproductive cells), or if the mutation occurred only during fertilization, the disease will not be considered hereditary.
Non-heritable genetic disorders
Down syndrome is an example of a non-heritable genetic disease. People with Down syndrome have three copies of chromosome 21 instead of two, in all or some of their body’s cells. This disorder causes various health impairments and a life expectancy that does not usually exceed fifty years of age. The cause for the addition of the third chromosome remains unclear, it is most likely caused by an adhesion of two chromosomes that occurs in older eggs.
The prevalence of Down syndrome birth in women over 45 is 1:19. Although the chromosome abnormality occurred in the mother’s egg because of a genetic default, the mother was not a carrier of the disease, did not inherit it from her parents and probably does not have siblings with this disorder. For these reasons, the disease is not hereditary.
Every hereditary disease is genetic, but not all genetic diseases are hereditary
Hereditary diseases are genetic disorders passed down from one generation to the next, or diseases with a potential for passing down between generations through a passing down of mutated genes from parents to their children.
Non-heritable genetic diseases can occur during fertilization, but also during later stages in life. Therefore, although the source for all genetic disorders is a genetic mutation, the relevant mutations are not always inherited, which means that not all genetic diseases are also hereditary.