That’s a big question. Genes are basically recipes for making proteins. Mutations are changes in the recipes. Your question is like saying “If there is a mistake somewhere in this cookbook, what will happen?” The answer depends on a lot of things.

If the mutation was in the genes they got from one of their parents, it would be in every cell in their body. What would actually happen depends on the mutation. It might make absolutely no difference because it was not a mutation in anything important or because the change did not make any difference, like using a different brand of sugar in your cookie recipe. Or maybe they can use the good copy of the recipe that they got from their other parent. Or it could be terrible, like using salt instead of sugar and both parents gave them bad recipes.

If the mutation happened during the person’s development (before they were born), it might affect some cells, but not others. Again, it might make no difference or a big difference, all depending on which cells had the mutation, which recipe had the mutation, and how bad it was. For example, the recipe for insulin is very important in controlling your blood sugar, but only certain cells in your pancreas make insulin. If the cells in your liver had bad insulin recipes, it wouldn’t matter.

If the mutation happened after birth, it would probably only start off in one cell. Things like sunlight or cigarette smoke can cause mutations, or mutations can happen for no reason. Most of these cells die or are attacked by the body’s immune system. Unfortunately, some of these mutations cause cells to divide like crazy and spread around the body. This is cancer. If a person is lucky, the body will destroy the cancer cells too. Chemotherapy and radiation therapy can help the body fight the cancer cells.

There is a whole lot we don’t know about mutation. If you are interested in questions like this, you may want to be a cell biologist or geneticist.