Genetic Mythbusters - Part I
In this post I thought I would start talking about some general genetics myths that I find many people share. First, the idea of diseases or disorders skipping a generation. From a genetic standpoint, there really is no such thing as skipping a generation. This is because you cannot pass on what you don't have. So if your father has, for example, a condition called neurofibromatosis, and your son has it too, it didn't skip you genetically. Your son would not have what your father had if it did not come through you. What's happening is that it's not being clinically expressed in you, for many different reasons. It is also possible that you are expressing it in such a mild form that it never came to medical attention, and is not a problem for you.
Probably an easier example to understand is hereditary breast cancer. This is a condition that can be inherited due to variations in many different genes. While BRCA1 and BRCA2 are the ones that most people have heard of, there are other genes that are involved in the inheritance of breast cancer. What many people don't realize is that the vast majority of cancers are not hereditary to begin with. Having cancer in your family does not mean that you will have it too, and not having it in your family does not mean that you will not. This is really a topic for another blog!
On the topic of skipping a generation, consider that a grandmother had breast cancer,and then sometime later her granddaughter was diagnosed with breast cancer also. Her daughter was healthy, at least so far. There are two points here: the first is that diagnoses occur at different ages, and so the fact that the daughter is unaffected does not mean that she won't develop cancer later in her life. But the second and more relevant point for this discussion is that whatever genetic variation there is in BRCA1, BRCA2, or a different breast cancer susceptibility gene, the daughter must have inherited it from her mother in order to pass it to the granddaughter. The granddaughter could not have inherited a gene alteration from her grandmother unless it went through her mother.
From a genetic standpoint, there really is no such thing as skipping a generation. This is because you cannot pass on what you don't have.Janice Berliner, 2019
The only other possibility is that the granddaughter inherited a gene alteration from her father, which brings up another myth. Breast (and ovarian) cancer can be and are inherited through males. Half the women who have hereditary breast and/or ovarian cancer have inherited it from their fathers. The fact that a man does not have ovaries does not mean he cannot pass a gene alteration for ovarian cancer from his mother to his daughter. He is far less likely to develop breast cancer than his mother or daughter, simply due to being male, but it is not impossible. When there are mostly males in a family, it can very much confuse the analysis of the family history. In other words, if you have no breast or ovarian cancer in your family, but there are very few females, this does not guarantee that there is not a hereditary cancer syndrome.
It might surprise you that only 10 - 15% of breast cancers are hereditary; therefore, the vast majority are considered sporadic. Having one relative with breast cancer, for example, especially if she was post-menopausal at the time of her diagnosis, does not mean you have a family history. This very likely does not represent something genetic. If you think you have a concerning family history of cancer, I would encourage you to seek genetic counseling to learn whether this might be something that could affect you. If you need to find a genetic counselor in your area, please contact nsgc.org and click on "find a counselor."