When a change in a gene is passed from parent to child. This happens when a gene changes in a way that is not normal inside the mother's egg or the father's sperm. The child that comes from this egg or sperm gets the gene change in every cell of the body. The change can be passed on from one generation to the next.
For more information, see Breastcancer.org's Genetics and Breast Cancer Risk page.
Audio contributed by: David Hyde Pierce