Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: An organism's traits are largely determined by the genetic makeup of its parents. A mutation in which kinds of cells in a parent could cause a new trait to appear in the parent's offspring?
sperm or egg
Gametes (sex cells).