Where in the body does a mutation have to occur in order to cause an inherited change in the offspring?

Answer

This has to be in germinal cells./germ-line cells. That is in the sex cells of either sperms or the eggs. it may occur during Gametogensis.

The fusions of these sex cells resulted in a new organism. Where the genes inherited in the two sex cells are fully expressed, It is duding the expression that the effect of the mutation will be shown.