Along the Way Mistakes can occur as the code is copied and re-copied when the cells divide. Sometimes this doesn't matter much, but sometimes it matters a lot. The sickle shape makes it hard for the blood cells to get around in the blood vessels. Another disease that is inherited in this direct way is cystic fibrosis. This is a disease in which mucus clogs the lungs.
Most diseases have a non-gene trigger.
That disease is PKU, in which protein is not eliminated but builds up. If protein foods are eaten, the brain will not fully develop. Our environment, our care in using chemical products, our personal habits and our diet and exercise can be important. And remember that the natural substances we encounter can sometimes change a gene. Scientists say that some of our genes were modified by conditions hundreds of years ago and were passed to us from our great, great grandparents.