Research in epigenetics would tell you that on both accounts, you're probably wrong. It used to be that scientists believed that DNA was the greatest determinant of biological behavior. Not anymore. Scientists such as B. Lipton, in his book, The Biology Of Belief, would say that we can't ignore the fact that chromosomes are comprised of DNA and regulatory proteins, and that environmental factors, from Cheerios, to cell phones, to dental amalgams and your older brother screaming at you, can powerfully alter these regulatory proteins for better or for worse, which in turn modifies your genetic expression, and hence celullar behavior.
How does this happen? Cells receive and process signals from the environment via receptors on the cell membrane, which regulates the exchange of nutrients/waste/energy inside and outside the cell. Under stress, these processes become compromised and altered, whether that stress is nutritional, electromagnetic, emotional or whatever. This results in a myriad of consequences, including the cell not receiving the nutrients and energy it needs for life, and/or not being able to efficiently shuttle toxins across the cell. The chromosomal regulatory proteins inside the cell's nucleus naturally suffer the effects, as well.
Additionally, consider that the body's biological processes are not carried out solely on the level of physical matter, but also via a complex network of energy fields within the body. One little disruption in these energy fields, whether it be from the energy generated by"thought," electromagnetic forces or others, can cause a cascade of dsyfunctions in the body, including that of affecting the cell membrane's receptor proteins and hence chromosomal regulatory proteins.
Bottom line? You aren't fated to be a victim of your heredity! You can alter your genetic expression through "positive" environmental influences, in the form of healthy, wholesome food, nurturing relationships, and a lifestyle that limits electromagnetic and other toxic exposures.