Q. I don’t understand gene mutations. Are mutations like the Philadelphia chromosome present from birth? If so how do people ever survive? Wouldn’t the mutation cause a crisis much sooner than adulthood?
A. That’s a great question! I love it because it’s one of those questions that can persist even after you sit through lecture, or do whatever reading you’re supposed to do for class. For whatever reason, it’s something you’re just expected to know, even though it’s not really explained outright. It’s great to identify and put those questions into words, so you can get an answer and clear things up now (rather than carrying that hole in your knowledge for years and years).
So, on to the question. No, mutations like the Philadelphia chromosome are not present from birth. They arise in one cell in the body (in this case, a cell in the bone marrow), and get passed on to all future descendents of that cell. The same is true of the t(8;14) mutation in Burkitt lymphoma , and many other mutations that occur in cancer.
This type of mutation is called a somatic mutation, and it can spontaneously arise in any cell in the body (“soma”) except germ cells (eggs and sperm) at any time during the patient’s life. This type of mutation is limited to the descendents of the original cell that developed the mutation – it is not present in other cells in the patient’s body. Since it’s not in the germ cells, it is not passable from parent to child. Somatic mutations can cause cancer or other diseases – but they can also be silent, or they may simply contribute to genetic diversity, which is a healthy thing for a species.
The other type of mutation that you allude to is a germline mutation. Germline mutations exist in the patient’s germ cells, so they can be passed on to future generations. A patient who inherits a germline mutation will have that mutation in all the cells of his or her body (since the mutation was present at conception). Like somatic mutations, germline mutations may be silent, may cause disease, or may generate beneficial genetic diversity.
So the bottom line is: somatic mutations develop after conception in any cell in the body, and are passed down only to descendants of that particular cell, not to future generations. Germline mutations are passed from generation to generation through the germ cells; they are present at conception and therefore are passed down into every cell in the body.