The definition of a germ cell is a cell in the gonads, (ovaries or testes) that gives rise to a gamete, egg or sperm cell. The process of gametogenesis is where a diploid cell in the gonads, testes or ovaries undergoes meiosis and this results in haploid cells, known as sperms or eggs, after differentiation. However, gametes usually aren't capable of surviving for long if fertilization doesn't occur. Also, spermatogenesis is a more rapid process and oogenesis(egg production) usually takes longer and is interrupted for periods of time. Therefore, the answer to your question is germ cell mutations occur in the ovaries or testes because germ cells are in the gonads and affect the sperm and ova or egg cells.