Final answer:
Germ cell mutations can have significant consequences in the next generation as they can be passed on to offspring and affect every cell in their body.
Step-by-step explanation:
Germ cell mutations can have significant consequences in the next generation. If a mutation is present in a germ cell, it can be passed on to offspring and every cell in their body will have the mutation. This is how hereditary diseases, such as Tay-Sach's disease and cystic fibrosis, are transmitted. Therefore, option D is correct as germ cell mutations are usually found in every cell of the next generation.